Immunomodulatory effects of different retinoids have been demonstrated, both in vivo and in vitro, in different cellular lineages including human and murine thymocytes, human lung fibroblasts, Langerhans' cells, tumoral cells and natural killer (NK) cells; however, any attempt to demonstrate the effect of retinoids on human peripheral blood mononuclear cells (PBMC) resulted in negative results. In the present work, it is shown that retinol and retinoic acid induce a marked increase of proliferation on human PBMC from 32 unrelated healthy individuals, which had previously been stimulated with anti-CD3 antibodies 48 hr before. Serum-free medium, specific retinoid concentration (10(-7) M) and a particular timing of retinol addition to the cultures (48 hr after CD3 stimulation) was necessary clearly to detect this retinol-enhancing effect. The increased proliferative response is specifically mediated via the clonotipic T-cell receptor-CD3 complex and correlates with the up-regulation of certain adhesion/activation markers on the T-lymphocyte surface: CD18, CD45RO and CD25; also Th1-type of cytokines (interleukin-2 and interferon-gamma) are found concordantly increased after retinoid costimulation, both measured by a direct protein measurement and by a specific mRNA increase. In addition, it is shown that the in vitro retinol costimulation is only present in immunodeficient patients who have no defect on CD3 molecules and activation pathway. The fact that retinol costimulate lymphocytes only via CD3 (and not via CD2 or CD28) and the lack of response enhancement in immunodeficients with impaired CD3 activation pathway indicates that retinoids may be used as therapeutic agents in immune system deficiencies that do not affect the clonotypic T-cell receptor.