Depressed antibody responsiveness to sheep erythrocytes in mice infected with Friend leukemia virus continued in vitro when spleen cell cultures from infected animals were cultured in the presence of antigen. Addition of PE cells from normal donor mice to the immunologically depressed splenocyte cultures resulted in a marked restoration of antibody responsiveness. Restoration of the immune response was PE cell dose-dependent; a ratio of 1 PE cell per 10 splenocytes resulted in the largest numbers of PFC's. These results suggest that impaired antibody responsiveness by spleen cell cultures from FLV-infected mice may be due, in part, to effects on antigen-processing macrophages, since restoration of immune responsiveness occurs by PE cell supplements.