A monoclonal antibody, MID 2, which reacts with an epitope common to all human leucocytes, was used to show that the leucocyte common antigen can be resolved into four glycoprotein components with molecular weights of 220 K. 200 K, 180 K and 160 K. The 200, 180 and 160 K peptides were all present to various extents in the T and B lymphoblastoid cell lines examined, but the 220 K component was only detected in the B‐cell lines. The 220 K glycoprotein was also lacking in human thymocytes, but evidence of its expression in peripheral blood T lymphocytes was obtained, suggesting that it might be acquired as the cells differentiated. The four glycoproteins isolated from tonsil cells gave similar patterns on peptide mapping by the Cleveland enzymatic method or by cyanogen bromide cleavage, indicating extensive homology. It is conceivable that they differ from each other by the acquisition of simitar repeating domain sequences of approximately 20 K. Alternatively, they may share a common peptide structure but differ in their electrophoretic behaviour in SDS gels owing to differences in carbohydrate; if this is the case, however, experiments with tunicamycin suggest that O‐linked oligosaccharides must be invol