The CD40L expressed on activated CD4+ T cells delivers contact-dependent proliferative and anti-apoptotic signals to B lymphocytes. Little is known about molecular mechanisms of constitutive expression of CD40L on some non-Hodgkin's lymphomas, especially about involvement of two signal pathways regulating its expression in normal cells; one involving calcineurin, and the other protein kinase C. We analyzed by flow cytometry the effects of 6-hour stimulation of both pathways (stimuli: PMA and ionomycin) and their inhibitors: cyclosporin A and chelerythrine, on CD40L expression. Two Jurkat clones differing in CD40L surface expression: clone 217.6 (CD40L-) and 217.7 (CD40L+) were studied. Our experiments showed that high level of CD40L expression on the surface of 217.7 cells was reduced after stimulation with PMA. The same effect was observed for combination of PMA and chelerythrine or for PKC inhibitor alone. In 217.6 cells, only chelerythrine used alone induced low level of CD40L expression, while PMA and ionomycin were without effect. These results suggest that CD40L surface expression is mainly dependent on protein kinase C activity. By using PepTag Assay we have confirmed that in both Jurkat clones PKC activity is higher than in normal blood lymphocytes.