Mitogen treatment of murine (BALB/c) B-cells induces two different endogenous retroviruses involving two unlinked, presumably proviral, loci Bxv-1 and Bdv-1. To determine the usefulness of these loci as genetic markers for B-cell differentiation their expression was studied under conditions that interfered with B-cell proliferation and differentiation into IgM-secreting plaque-forming cells (p.f.c.). Maximum production of both viruses followed peak DNA synthesis by an interval of about 18 h. Treatments that blocked DNA synthesis or killed proliferating cells inhibited virus production. Addition of BUdR to mitogen-stimulated cultures selectively induced Bxv-1 while inhibiting the generation of p.f.c. Both effects require BUdR incorporation into the DNA of proliferating cells. 5-Azacytidine induced Bxv-1-dependent virus production without inhibiting terminal B-cell differentiation. Pretreatment of mitogen-stimulated B-cells with anti-mouse IgM serum decreased both virus production and generation of p.f.c., but had little effect on DNA synthesis. Experiments using a mitogenic F(ab')2 preparation of anti-IgM in the presence and absence of lymphokines also suggested that the generation of p.f.c. and Bxv-1-dependent virus production are linked phenomena. The data imply that Bxv-1- and Bdv-1-dependent virus production require DNA synthesis and cell proliferation and, at least for Bxv-1, B-cell differentiation. It is proposed that the induction of these loci reflects the involvement of neighbouring DNA sequences in B-cell proliferation or differentiation.