Infection with Rowson-Parr virus (RPV) induced a rapid reduction in the number of immunoglobulin-positive and theta antigen-positive cells detectable by immunofluorescence in the spleens of susceptible BALB/c mice. The changes produced by RPV infection in the lymph nodes were different, since the number of immunoglobulin-positive cells was increased and the proportion of theta-positive cells remained unchanged. However, the ability of immunoglobulin-bearing cells to redistribute their receptors into caps was reduced in both types of lymphoid tissue. A similar pattern of changes was produced by infection with Friend leukemia complex, from which RPV was originally obtained. These effects of RPV and Friend leukemia complex may contribute to the immunodepressed state of infected mice.