The effect of neonatal infection with Friend virus (FV) and Rowson-Parr virus (RPV) on the maturation of the capacity to respond to sheep red cells, as measured by the numbers of hemolytic plaque-forming cells in the spleen, was investigated in BALB/c mice. Both viruses affected immunological maturation but there were significant differences between their effects. The development with age of the ability to produce plaque-forming cells in response to antigen was virtually abolished by FV and only slightly impaired by RPV. Furthermore, FV also suppressed the development of background plaque-forming cells, whereas RPV did not.