Evidence is presented which supports the phenomenon of heterogeneity amongst H-2Db-encoded Class I molecules. Two monoclonal antibodies (MoAb) called H141-31 and B22-249 were used in these studies. Both bind to the 'private' H-2.2 site of H-2Db-encoded molecules, but the binding of B22-249 is determined by carbohydrate moieties, whereas H141-31 appears to bind to a protein-defined epitope. Some H-2Db molecules, identified by the H141-31 MoAb, are primarily expressed on B lymphocytes and not T lymphocytes in spleen. The number of H-2Db molecules which bind H141-31 on B cells was also found to be three- to four-fold less than the number which bound the B22-249 MoAb. B cells of two mutant strains of mice, B6-C.H-2bm13 and B6-C.H-2bm14 which harbour very few nucleotide changes in the H-2Db gene, also show marked reduction in the binding of both antibodies. This suggests that a single common gene encodes both target molecules and that post-translational modifications such as differential glycosylation may account for heterogeneity amongst H-2Db molecules. This would explain the presence of the different H-2Db molecules defined here. It follows that differences in glycosylation evidently occur both within the B cell population, since H141-31 binds to only a subset of H-2Db molecules on B cells, and between T and B lymphocytes, since resting T cells do not bind H141-31 MoAb.