Ureaplasmas from various animal species were serologically characterized by the growth inhibition test as well as by the metabolism inhibition test. The animals positive for ureaplasmas included 9 common squirrel monkeys, 9 green monkeys, 164 cynomolgus monkeys, 58 chimpanzees, 73 goats, 11 sheep, 102 dogs, 116 cats, 64 cattle, 290 chickens, 3 red jungle fowls and 5 Japanese bantams. Simian ureaplasmas were divided into four serogroups in accordance with the zoological classification of the primates' families. Caprine ureaplasmas were all similar antigenically, and ovine ureaplasmas cross-reacted with the caprine ones, except for one strain. Canine, feline and bovine ureaplasmas were divided into groups of four, two and three, respectively. Avian ureaplasmas contained only one serogroup. All these ureaplasma serogroups were remarkably unrelated antigenically to each other, as shown by the growth inhibition test. Ureaplasmas could not be isolated from rhesus monkeys, Japanese monkeys, orangutans, gorillas, mice, rats, mongolian gerbils, hamsters, guinea pigs, rabbits, opossums, pigs, quails and turkeys.