A necropsy survey of inflammatory cellular infiltration associated with atherosclerosis showed that the degree of inflammation correlated with the severity of the atherosclerotic plaque and was greater in the presence of medial disruption. Sections were stained for IgG and IgM using the immunoperoxidase technique. Advanced plaques with medial disruption showed staining of the cytoplasm of plasma cells in the adventitia with IgG and IgM antisera. IgG was also demonstrated in the atheromatous material. The immunoglobulin production may have been stimulated by antigens formed in the advanced lesion. Such an allergic reaction might explain some of the complications of advanced atherosclerosis.