The CD27 membrane antigen is exclusively present on a large subset of human peripheral blood T lymphocytes and on mature thymocytes. Several observations point towards a specific role of this molecule on activated T cells. Upon T cell activation induced via the T cell receptor complex, CD27 expression greatly increases, while addition of anti-CD27 monoclonal antibodies amplifies the proliferative response. Within the CD4+ subset, only CD27+ T cells provide helper activity for B cell differentiation. Interestingly, CD27 expression differs not only quantitatively, but also qualitatively between resting and activated T cells. On resting cells, CD27 is a disulfide-linked homodimer with subunits of 55 kDa molecular mass. Upon activation, also a 55-kDa monomer seems to occur, while in addition a 32-kDa component is found. The relationship between these two proteins has been investigated. The 55-kDa and 32-kDa molecules do not seem to be physically associated. The two CD27 components are structurally highly homologous as determined by two-dimensional mapping of tryptic peptides. They both express the epitopes recognized by anti-CD27 antibodies, indicating that the 32-kDa molecule is also T cell specific. Both 55-kDa and 32-kDa molecules carry N-linked carbohydrate groups. However, they differ in other, not yet specified, post-translational modifications, and do not arise from a common precursor simply by alternative N-glycosylation. The 32-kDa form may be derived by proteolytic processing from the 55-kDa protein, but most likely not from a larger (common) precursor. Alternatively, both molecules may arise from a common gene by alternative mRNA splicing, or be derived from highly homologous genes. The dramatic change in molecular composition of CD27 suggests a newly acquired function for CD27 on activated T cells.