The CD31 antigen, a member of the immunoglobulin superfamily with a possible cell adhesion function, is expressed on approximately 50% of periperal blood lymphoid cells at relatively low intensity (10–20% of the level on monocytes). In the accompanying paper we showed that a mAb, 5A2.G5, which identifies a glycosylation‐dependent epitope of the CD31 antigen, bound to fewer lymphocytes than two other CD31 mAb, B2B1 and 2BD4, although the 3 antibodies bound equally well to monocytes. We have now analyzed the pattern of expression of epitopes of the CD31 antigen on lymphoid cell subpopulations using two‐color immunofluorescence and flow cytometry. Large granular lymphocytes (CD16⁺), CD8‐positive T cells and B cells (SMIg⁺) were mostly CD31‐positive as indicated by the binding of mAb B2B1 and 2BD4. Single populations displaying some overlap with the negative control were obtained in each case. In contrast, CD4‐positive T cells fell into two discrete populations with respect to CD31 antigen expression. mAb 5A2.G5 displayed weaker binding to all lymphoid cell types, indicating that the pattern of glycosylation of the CD31 antigen differs between lymphocytes (of all types) and cells of the myeloid lineages. The heterogeneity of CD31 antigen expression by CD4‐positive cells was further examined by dual‐labelling of purified CD4 cells with mAb B2B1 and CD45RA or CD29 mAb which identify naive and memory T cells respectively. The CD31 antigen was found to be preferentially expressed by the CD45RA‐positive, naive cell population.