Concentrations of a fragment of the c-erbB-2 translational product (p185 fragment) were measured in serum of 70 breast cancer patients, 19 healthy blood donors, and 18 pregnant women using a heterogenic enzyme immunoassay. The serum concentrations of blood donors and pregnant women were below 30 kU/l. Breast cancer patients showed serum concentrations up to 578 kU/l. All 9/70 patients with serum concentrations higher than 30 kU/l had clinical evidence of metastatic disease and the serum levels of all 35/70 patients without metastasis lay within the normal range. From 9/37 patients with p185 overexpression of the primary tumor in immunohistochemical analysis 3/9 patients with metastatic disease had elevated serum levels higher than 30 kU/l. In all, 6/9 patients without metastasis serum levels were below 30 kU/l. The data of the present study suggest that determination of serum p185 fragment concentrations may be useful as a diagnostic tool in postoperative follow-up of breast cancer patients with c-erbB-2 overexpression of the primary tumor.