The level of a c-erbB-2 related protein was determined in sera from 168 breast carcinoma patients, 12 females with benign breast disease, and 66 female controls using an ELISA (enzyme linked immunosorbent assay) kit. Elevated c-erbB-2 related protein level was detected in one of 13 preoperative sera (8%), two of 62 postoperative sera from patients without recurrent disease (3%), and 55 of 93 sera collected at recurrent disease (59%). Elevated serum levels were detected significantly more often in patients with distant metastases than in patients with recurrent disease restricted to loco-regional areas (68% versus 19%). Presence of elevated serum level was associated with ERBB2 gene amplification and c-erbB-2 protein overexpression in tumour. None of the patients who had normal ERBB2 gene copy number in tumour had elevated serum levels. Although the usefulness in postoperative prediction of the presence of micrometastases is somewhat questionable, the results suggest c-erbB-2 related protein to represent a novel tumour marker in serum and other body fluids from breast cancer patients at the time of diagnosis and during treatment monitoring.