Our previous report revealed the production of monoclonal antibody (MoAb) CDI 315B by immunization of mice with tumor extract proteins of human invasive ductal breast carcinoma. In the present study we report on the immunocytochemical reactivity of this MoAb with formalin or methacarn fixed, paraffin embedded tissue sections and also with cell cultures. Among breast tissues, positive staining was detected in 88% (64 of 73) of primary breast carcinomas, 77% (7 of 9) of metastatic lymph nodes, 24% (8 of 33) of benign breast disease and 15% (2 of 13) of normal breast tissue. No immunostaining was detected with several other tumors, with the exception of melanoma, where 63% (5 of 8) of positive staining was found. On in vitro cell lines, positive reaction was detected only with breast carcinoma and melanoma cells, but not with other examined cell lines. On benign breast disease tissue sections, positive reaction was detected in areas with cell hyperplasia. On normal breast tissue sections MoAb 315B stained the epithelial cells of terminal ductuli. Since the MoAb 315B recognized some antigen present in the cytoplasm of most breast carcinoma cells, this MoAb may have potential application in diagnosis and management of breast cancer.