The response of lymphocytes of M. arthritidis- or A. laidlawii-infected rats to PHA and ConA was studied. Mycoplasma infection was shown to stimulate the DNA synthesis in lymphocytes of the spleen and lymph nodes and to depress the responses to mitogenes one week after infection. By the same time splenocytes of A. laidlawii-infected rats were shown to only slightly proliferate in response to PHA. After 4 weeks of infection the lymphocyte transformation was greatly increased in both mycoplasma- and acholeplasma-infected rats. The first to be stimulated by the mitogenes were cells from the lymph nodes followed by those from the spleen. By the 65th day of infection no stimulation occurred. Possible mechanisms of the phenomena described are discussed.