B-lymphocytes obtained from patients with either rheumatic fever or rheumatic heart disease and from normal subjects were reacted with serum obtained from rabbits immunized with streptococcal cell wall antigen. The presence of cytotoxicity was sought using an inverted phase microscope after differential uptake of eosin dye. The serum was found to be significantly more cytotoxic to HLA-DR4 containing cells of both patients and normals compared with DR4 negative cells (P less than 0.0001).