We studied c-erbB-2 and c-erbA-1 (ear-1) gene amplification, and c-erbB-2 protein expression in 123 primary Japanese breast cancers. c-erbB-2 amplification was found in 19 of the 123 tumors (15%), and c-erbA-1 was coamplified in 7 of the 19. The presence or absence of c-erbB-2 amplification correlated with the grade of cellular atypism (P = 0.008), or that of mitotic index (P = 0.002), but not with the histologic types. The tumor size (P = 0.04) and the lymph node status (P = 0.06) were associated, but the patients' age, the TNM stage, or the presence or absence of estrogen or progesterone receptors was not associated, with c-erbB-2 amplification. There were no differences in the histologic type, cellular atypism, mitotic index, and other disease parameters between tumors with c-erbB-2 amplification only and those with coamplification of c-erbB-2 and c-erbA-1. Paraffin sections from all 19 tumors with c-erbB-2 amplification, and those from only one of 104 tumors without the amplification were positively stained with polyclonal anti-c-erbB-2 protein antibody. Since the correlation between the amplification and the protein expression was excellent, such immunohistochemical studies may be substituted for the time-consuming DNA studies using Southern blotting.