Fetal bovine serum (FBS)-supplemented cell culture medium has been the standard medium used in assays of murine lymphocyte proliferation. While FBS allows cell growth and viability, it requires screening and contains uncharacterized elements which may yield inconsistent results from batch to batch. It also may include growth factors and immunologically reactive proteins that obscure assays for specific responses because of high background proliferation. Recently, chemically defined serum-free media have become available for human lymphocyte culture. We compared several of these media against FBS-supplemented medium and found that one of them, AIM V (Gibco), produced a marked increase in the ratio of positive to background counts in [3H]thymidine incorporation assays of antigen-specific proliferation. Similar results were obtained when responses to mitogenic lectins were examined. AIM V supported proliferation of lymphocytes in a variety of haplotypes, it supported MHC-specific proliferation in response to soluble antigens in a manner consistent with previous reports, and it supported proliferation in response to alloantigen as displayed in the mixed lymphocyte culture. A high ratio of positive to background counts was consistently observed. The results indicate that this serum-free medium enhances the analytic power of proliferation assays.