The occurrence of the basement membrane (BM) glycoprotein laminin was investigated in 91 malignant breast tumors and in 359 axillary lymph nodes of which 70 contained metastases. The indirect double-layer immunoperoxidase technique was applied using rabbit anti-rat laminin. Positive cytoplasmic staining for laminin was found in all breast carcinomas. BM could be demonstrated around in situ carcinomas and in the higher differentiated carcinomas around islands of infiltrating tumor cells. Lymph node metastases resembled the primary tumors in the cytoplasmic staining reaction for laminin, but no BM staining was found. Tissue from 50 benign breast lesions showed cytoplasmic staining for laminin in both the secretory and the myoepithelial cells, but consistently weaker than in the carcinomas. BM was always present.