In vitro induction of endogenous type C viruses from normal mouse spleen cells by Escherichia coli lipopolysaccharide (LPS) and combination treatment with concanavalin A and 5-bromo-2''-deoxyuridine (Con-A/BrdU) was previously reported. To identify the cell types sensitive to virus induction, and to study the relationship of mitogenicity to virus induction, T cell populations (BALB/c thymus cells and cortisone resistant thymus cells), B cell populations (nu/nu [nude mouse] spleen cells and lymph node cells), adherent BALB/c peritoneal cells and mixed populations (BALB/c spleen cells, macrophage depleted BALB/c spleen cell and lymph node cells) were compared. LPS induction occurred only in B cell containing populations. Induction by Con-A/BrdU depended on the presence of T and B cells. In both instances, neither macrophages nor hemopoietic cells appeared to be a major source of virus. Treatment with rabbit anti-Ig [immunoglobulin] serum and complement reduced virus induction by LPS/BrdU but not by Con-A/BrdU, suggesting that different cell populations produce virus after stimulation with these 2 mitogens. The expression of endogenous viral genomes plays a decisive role in the development of spontaneous leukemia.