Fifteen young (47-81 days old) and 5 adult (360 days old) Rowett nude rats (rnu/rnu) and 12 euthymic littermates (rnu/+) (56 days old) were infected with Mycoplasma (M.) arthritidis ISR1. The polyarthritis developing after the infection was more severe in the athymic than in the euthymic rats. In the nude rats the disease was progressive and led to the death of 6 animals whereas in the euthymic littermates the disease was resolved within 70 days after infection. In contrast to euthymic rats athymic animals did not develop antibodies against M. arthritidis within 6 weeks (young nude rats) or 23 weeks (adult nude rats) after infection, indicating that M. arthritidis is a thymus dependent antigen. However, low antibody titers were found in the nude rats infected at an age between 47 and 81 days at 340 days after infection, probably due to the development of T-cells which occur in older nude rats. The euthymic as well as the young athymic rats showed an increase in antibody titers after a challenge infection 345 days after the first infection and both were resistant to challenge infection. On the contrary the adult nude rats infected with M. arthritidis did not develop antibodies against the mycoplasmas and were highly susceptible to a second infection after 198 days. This indicates that antibodies play an important role in the defence of the M. arthritidis infection in rats.