Cell membranes of Streptococcus mutans BHT serotype b were prepared after glass bead disruption or mutanolysin digestion of whole cells. Immunoblot analyses of BHT membrane extracts revealed major polypeptides of 42,000, 46,000, 62,000, and 82,000 daltons, as well as several minor bands, to be reactive with rabbit anti-human heart immunoglobulins. Heart cross-reactive antigens have been reported in the cell walls and culture fluids of several S. mutans serotypes. This represents the first report of cell membrane-localized heart cross-reactive antigens in this oral pathogen. Positive enzyme-linked immunosorbent assay and immunoblot reactions were also obtained with heart tissue antigen and anti-BHT sera, indicating mutual cross-reactivity. The major cross-reactive component detected by immunoblotting of human heart extracts was a 69,000-dalton polypeptide.