The CD31 molecule is a leucocyte-surface glycoprotein of 130 kDa with homology to the immunoglobulin gene superfamily. In this study we report on the expression of CD31 on human T cells and demonstrate that it subdivides peripheral blood T lymphocytes into two novel subsets. CD31 is expressed by 38% of CD3+ lymphocytes. About 85% of CD31+ T cells display the CD45RA phenotype, 35% the CD45RO phenotype, 24% the CD4 phenotype and 72% the CD8 phenotype. There is also a correlation between CD31 expression and CD45RA expression in cord blood T cells; 89% of CD3+ cord blood cells express CD31, and most of them have the CD45RA phenotype. A discrepancy was found with thymocytes, which are positive for CD31 but negative for CD45RA. Stimulation of human T cells leads to down-regulation of CD45RA, while CD31 continues to be expressed. In functional studies, CD31 antibody binding to T lymphocytes does not lead to mobilization of intracellular calcium, proliferation or modulation of T-cell proliferation.