CD40 and CD43 arc two cell-surface glycoproteins that appear to be functionally involved in the growth stimulation of human B cells. Whereas CD40 is structurally similar to the NGF receptor and is present on all resting B cells. CD43 displays no homology to other known proteins and is expressed only on a subpopulation of these cells. To further understand the extra- and intracellular signals regulating these molecules and in which stage of activation they may play a role, we used various activation strategies and studied their expression on tonsillar B cells. As expected, activation of protein kinase C by TPA increased both CD40 and CD43. In contrast, a rise in intracellular Ca2+, e.g. by ionomycin, did not influence the expression of these antigens. However, in the presence of TPA, ionomycin further up-regulated CD43 but not CD40. Anti-IgM behaved similarly to ionomycin suggesting that the effect of this reagent was due primarily to its ability to increase intracellular Ca2+. Of three interleukins(lL-2. IL-4 and IL-6) only IL-4 had a significant effect when used alone in that it up-regulated CD40 but not CD43. However, in Ihe presence of anti-IgM, both IL-2 and IL-4 synergistically up-regulated the two antigens. Complementation of antigen receptor stimulation with TPA or IL-4 increased CD4U during the first 24 h. whereas up-regulation of CD43 did not occur until 24 to 4S h after stimulation. With regard both to up-regulation in response to different stimuli and to kinetics, CD40 expression paralleled that of the early activation antigen CD23, whereas CD43 was induced in parallel with the transferrin receptor (CD71). Taken together, our results suggests that the expression of CD40 and CD43 is regulated by different intracellular signals and that CD40 may be important during early activation, whereas CD43 may have its major function during later stages of B-cell differentiation. These assumptions are in line with the observations that CD4U antibodies can directly activate resting B cells and that CD43 are retained on plasma cells.