Class II antigens of the major histocompatibility complex (MHC) are normally expressed only by cells of the immune system. They may, however, also appear on other cells, both in vivo and in vitro. We have studied class II MHC antigen expression on human arterial smooth muscle cells using immunocytochemical analysis of surgical biopsies. The class II antigens, HLA-DQ and HLA-DR are virtually absent from cells of normal arteries, but appear in atherosclerotically transformed tissue, where the majority of cells express HLA-DR, and one-third of the cells express HLA-DQ. These atherosclerotic plaques are composed of a heterogeneous population of cells, which includes smooth muscle cells, macrophages, and T cells. Among smooth muscle cells containing the muscle-specific intermediate filament protein desmin, one-third of the cells express HLA-DR and almost as many express HLA-DQ. These cells also contain the invariant gamma chain, which is associated with class II antigens during intracellular transport. Plaque macrophages, in contrast, are usually of the DQ-DR+ phenotype. The patterns of class II antigen expression are discussed in relation to cell differentiation and pathogenesis of disease.