CD4 ligand binding to the CD4 molecules has been shown to inhibit T cell proliferation and IL-2 transcription and synthesis. We have recently shown that this inhibition correlated with a CD4-mediated inhibition of the kinase Erk-2 and c-Jun-N-terminal kinases (JNK) which play a key role in IL-2 transcription. Moreover, we have previously reported that antigen-independent adhesion of CD45RObright/CD4+ T cells to B cells is negatively regulated by CD4 ligands, whereas that of CD45RAbright/CD4+ naive T cells is not. Other groups have described, in murine models, a differential sensitivity of memory and naive T cells to CD4-mediated inhibitory effects on T cell activation. The aim of the present report was to study the sensitivity of the naive and memory CD4+ T cell populations to the CD4-mediated inhibition of Erk-2 and JNK activation. Our data show that preincubation with anti-CD4 mAb, of the CD45RAbright/CD4+ naive and the CD45RObright/CD4+ memory human T cell populations, induces inhibition of both Erk-2 phosphorylation and Erk-2 activation by phorbol ester or anti-CD3 mAb. In contrast, CD3 mediated JNK activation was inhibited in the memory but not in the naive CD4+ T cell population, whereas JNK activation by phorbol ester or phorbol esters plus Ca2+ ionophore was inhibited by anti-CD4 mAb in both T cell populations. These data further demonstrate a differential sensitivity of naive and memory CD4+ T cell populations to the CD4-mediated negative signaling.