Interaction between the B cell glycoprotein CD40 and its ligand (CD40L), expressed by activated T cells, is of crucial importance in the generation of specific antibody response, which is impaired in HIV+ individuals. We studied the expression of CD40L by lymphocytes activated with PMA plus ionomycin in 17 HIV+ individuals and 12 healthy donors. In HIV+ individuals, the percentage of cells expressing CD40L was lower than that in the controls (22.6 +/- 14.7 vs 40.1 +/- 12.0; P < 0.002) and clearly correlated with that of CD4+ peripheral blood lymphocytes (r = 0.83; P < 0.001); therefore, the reduced CD40L expression might be explained by the decrease of the CD4+ cells. In fact, the expression of CD40L by purified CD4+ cells was comparable in individuals with and without HIV infection. These data indicate that the ability of CD4+ cells from HIV individuals to express CD40L is not impaired, at least after optimal stimulation in vitro.