Pretreatment serum levels of squamous cell carcinoma (SCC)-antigen as determined with a polyclonal and monoclonal antibody-based assay were determined in 136 patients with invasive mammary carcinoma, 8 patients with ductal carcinoma in situ and 23 patients with benign breast diseases. For the polyclonal assay, the overall sensitivity was 44.8% at a cut-off level of 2.0 ng/ml and 25% for 2.5 ng/ml. For the monoclonal assay, the sensitivity was 11.0 and 25.0% at cut-off levels of 2.0 and 1.5 ng/ml, respectively. On the basis of analysis of specificity of the two assays for the benign control group, the cut-off level of the polyclonal assay has to be 2.5 ng/ml in the case of mammary carcinoma, of the monoclonal assay it has to be 2.0 ng/ml. For 175 serum samples of the whole group of patients (pre- and postoperative samples), the polyclonal RIA (x) and the monoclonal IRMA (y) were measured. The orthogonal regression analysis gave the equation: monoclonal IRMA = 1.03 polyclonal RIA - 1.15 ng/ml (Sy.x = 0.48; r = 0.7225). We also came to the conclusion that the SCC-antigen measured by either assay is different.