Woodblock prints always seemed such a safe, innocent kind of art, perfect for thank-you notes to elderly relations. Those Edo street scenes, with the little people teeming over bridges in their pointy rain hats. Those pretty views of Fuji and the stations of the Tokaido. Even Utamaro's pictures of the "beauties" of the Yoshiwara pleasure quarter are tastefully indirect, and imply much more than they show. Admire the slightly disheveled looking lady in the loose-belted kimono. Never mind that she is actually a prostitute fresh from a vigorous encounter with a customer.

This autumn, the British Museum held a show which destroyed forever the image of ukiyo-e as genteel whimsy. Entitled "The Passionate Art of Utamaro Kitagawa," it represented the most comprehensive exhibition yet of the work of the great 18th-century print maker. But crucially it displayed for the first time ever a large number of shunga, one of the most popular ukiyo-e genres, and one which has been almost effaced from view in the last 100 years. The word literally means "spring pictures" but would be more accurately rendered as "mucky prints." Shunga are simply detailed depictions of men, women and fish, with huge genitals, having enthusiastic and acrobatic sex with one another. Explicit is not the word. Untroubled by the laws of gravity or probability, Utamaro's heroes and heroines get up to things that would leave devotees of Hustler and Screw magazine gaping in admiration.

A pearl diver is ravished by an amphibious kappa water sprite. A young naked woman swoons in the slithering embraces of a randy octopus. A maid lends a hand as her mistress is taken from behind by a man with a phallus longer and thicker than his arm. Shunga are filthy, but they are also extremely beautiful. There is a marvellous attention to detail--to the drapes of the couples' kimonos, to the ecstatic curl of a lady's toes, but above all to the act of penetration, and to the engorged genitals which throb on every plate. The concluding shunga of the 12-print folios was often very simple: a yawning pudendum, in close up, covering the entire page. (These proved too strong even for the British Museum.)

For the last month, the exhibition has been showing at the brand new Chiba City Museum of Art (until Dec. 16). But guess what? No shunga. Quietly and invisibly, the most popular part of the London exhibition has been excised. In Utamaro's own country, there has never been a large scale public exhibition to include some of his most important work.

It's unwise to get culturally uppity about this. It wasn't until the early 1970s that the first ever display of shunga, at London's Victoria and Albert Museum broke the taboo, paving the way for the reproduction of the prints in the catalogues of Sotheby's and Christie's. Similar taboos have dogged the reproduction of the pictures. Illustrated volumes appeared in Germany as early as 1903--but Britain and America have not been so laid back. Even now, there are no more than a handful of illustrated books on shunga; in 1953, the U.S. publication of Ginchner's Erotic Aspects of Japanese Culture prompted police raids, although it eventually appeared uncensored.

In Japan, the ambiguities of the censorship laws allow dealers, collectors and scholars discreetly to circumvent them. The law forbids the buying and selling, but not their ownership or private display. Until recently, books of shunga were sanctioned as long as the sex organs were obliterated. Then three years ago, several of Japan's top art historians, including Shunjo Asano, co-curator of the British Museum show, took the plunge and issued a definitive and unexpurgated four-volume collection. Privately advertised by flier and sold by subscription, the 2000 copies of the series went quickly and is now hard to track down. One volume, each containing 12 books of 12 plates each, changes hands for about Y200,000.

But the issue remains a sensitive one, wreathed in that peculiarly Japanese capacity for self-censorship. Collector Paul Binny spotted a print on display at the Togo Shrine market with the genitals discreetly covered by the vendor's paperweight. Further up the ladder, museums are quietly adding to their collections (even the Ministry of Education is reputed to be brightening up its vaults). A Yokohama museum announced that it would exhibit its shunga, but they were kept in a locked gallery, to which visitors were admitted only on request. "We're all in favor of showing shunga," says David Caplan, an ukiyo-e collector and proprietor of Mita Arts in Jinbocho. "But no one wants to have their head chopped off."