Mixed Rauscher leukemia virus (RLV) and M. arthritidis infection of (C57BL/6xA/Sn)f1 mice-hybrids, highly resistant to RLV, was accompanied by a progressive inhibition of rosette-forming cells (RFC) and plaque-forming cells (PFC), resulting in the induction of malignant erythroblastosis identical by cytology to Rauscher leukemia. The mice-hybrids infected with A. laidlawii and RLV developed significant splenomegaly on the 21st day of the infection, and their immune response was almost entirely suppressed, but both RFC and PFC populations as well as the spleen weight returned to the initial level by the 62d day the infection. A possible role of mycoplasm in the induction and development of Rauscher leukemia is discussed.