Naive and primed αβ T cells can be distinguished on the basis of their differential expression of CD45RA and CD45RO, respectively. The present study indicates that these CD45-isoforms also identify naive and primed maturational stages of γδ T cells and natural killer (NK) cells. In peripheral blood, all Vγ9-Vδ2 γδ T cells reportedly express CD45RO whereas all Vδ γδ T cells lack CD45RO. Here, we show that these CD45RO− Vδ γδ T cells all express CD45RA and the CD45RO+ Vγ9-Vδ2 γδ cells lack expression of CD45RA. The Vδ T cells acquired CD45RO expression and lost part of their surface CD45RA, following in vitro activation with phytohaemagglutlnln or IL-2. Also the CD3−CD16+ NK cells in peripheral blood that are uniformly CD45RA+CD45RO− completely converted to the CD45RA−CD45RO+ phenotype upon in vitro activation. Moreover, all cloned Vγ9-Vδ2 and Vγ1 T cells and NK cells express CD45RO and lack expression of CD45RA. Our results strongly suggest that CD45RA and CD45RO are genuine markers for naive and primed lymphocytes that represent distinct differentiation lineages.