Different monoclonal antibodies detecting the leucocyte common antigen have been obtained, for CD45 (GRT2, GRT3 and GRT4) and for CD45R (GRT22). Several epitopes have been defined with these monoclonal antibodies (MAbs). We have performed a comparative study with CD45 and CD45R MAbs on NK and T cell proliferation. Common epitopes of CD45 antigen were found to be involved in blocking of NK activity but not CD45R restricted determinants. In the cell proliferation assays, fresh human peripheral blood mononuclear cells were stimulated with PHA and soluble CD3 MAbs. CD45 and CD45R MAbs failed to demonstrate the capacity to modify the mitogenic response when optimal and suboptimal doses of PHA were used. In contrast, both (CD45 and CD45R) MAbs led to a significant rise in anti CD3 response. A CD18 (GRF1) was used as control and inhibited both PHA and CD3 T cell proliferation as well as NK activity. We think these results can be explained on the basis of different activation pathways (PHA versus anti CD3) and the accessory signals induced by these MAbs as recorded only in anti CD3 induced mitogenesis.