Retinoic acid (RA), an active metabolite of vitamin A, may synergize with interferons (IFN) to evoke a heightened immune response, suggesting combination therapy as a promising treatment for various cancers. Recently, we demonstrated a strong synergism between RA and polyriboinosinic : polyribocytidylic acid (PIC), an inducer of IFN, on antibody production in immunocompromised vitamin A-deficient animals. In the present study, we examined whether this combination could potentiate T-cell-dependent antibody production in non-immunocompromised rats. Forty male Lewis rats were treated with 100 microg all-trans-RA, 20 microg PIC, or the combination in either an 11-d study to evaluate antibody production, changes in lymphocyte populations, and cell proliferation, or a 21-hr study to evaluate early changes in lymphocyte populations and gene expression. The combination of RA + PIC significantly potentiated anti-tetanus IgG levels (P < 0.002). Similarly, this combination also increased the numbers of B cells and major histocompatibility complex (MHC) class II+ cells in spleen and lymph nodes, and natural killer (NK) cells in spleen and blood (P < 0.05). RA + PIC-treated rats had significantly higher levels of interleukin (IL)-10, IL-12, and signal transducer and activator of transcription-1 (STAT-1) mRNA (P < 0.05), and STAT-1 protein (P < 0.02). Treatments administered in vivo significantly modulated T-cell proliferation to anti-CD3/phorbol myristyl acetate + IFN-alpha ex vivo. These changes in antibody production, cell distribution, cytokine gene expression, and T-cell proliferation suggest that the combination of RA + PIC stimulates humoral and cell-mediated immunity, and deserves further testing in models of cancer chemoprevention in vivo.