A series of 21 male breast carcinomas were immunostained using NCL-CB11, an antibody directed against the internal domain of the c-erbB-2 transmembrane oncoprotein. In contrast to female breast cancer, where up to 35% of cases show positivity, all of these cases were negative. This suggests that no prognostic information regarding patient survival can be made in these patients and that male breast carcinomas may be under different growth control mechanisms from female breast carcinomas.