The effect of retinoid administration on the antigen presenting function of mouse dendritic cells (DC) was assessed. Culturing spleen cells with retinoic acid (10(-10)-10(-4) M) had no effect on the numbers of DC separated from these cultures. However, DC isolated from retinoic-acid-treated cultures were less stimulatory than DC from untreated cultures when added to allogeneic lymphocytes in a mixed leucocyte culture. Allogeneic stimulation by DC was also inhibited by pulsing separated DC with retinoic acid (10(-6)-10(-4) M) for 2 h. Pulsing DC with lower doses of retinoic acid (10(-14)-10(-20) M) enhanced this response. DC isolated from animals maintained on VAA-enriched diets had a reduced capacity to stimulate allogeneic lymphocytes. The response of unseparated lymph node cells pulsed with retinoic acid to untreated allogeneic DC was inhibited by 10(-10)-10(-4) M retinoic acid but enhanced by lower doses (10(-14) M). However, the inhibitory effect of retinoids on the function of responding lymphoid populations was abolished on removal of DC from responding cells. The results indicate that immunomodulation by retinoids could occur via an effect on the efficiency of antigen presentation by DC.