We report here both the range and patterns of reactivity of an IgG1 monoclonal antibody, B72.3, prepared against human, metastatic mammary carcinoma cells. When the avidin-biotin complex (ABC) immunoperoxidase technique was used on tissue sections, monoclonal B72.3 reacted with 19 of 41 (46%) primary mammary carcinomas and 13 of 21 (62%) metastatic lesions, either in axillary lymph nodes or at distal sites. Variable concentrations of antigen, recognized by B72.3, were observed among mammary tumors, as well as among different cell populations of a given tumor mass. Several patterns of antigen distribution were observed: membrane, diffuse cytoplasmic, focal and marginal. No reactivity was observed to normal mammary epithelium, stroma, or lymphocytes of the breast, nor to any cell types in a variety of other normal human tissues, melanomas, and sarcomas. Reactivity with all of four colon carcinomas was also observed. Assay of serial sections of mammary carcinomas with B72.3 and a monoclonal antibody directed against carcinoembryonic antigen demonstrated that these antigens were both distinct and non-coordinately expressed.