Two monoclonal antibodies (mAbs), designated MLuC5 and MLuC6, were produced against a human small cell lung carcinoma cell line. They were found to exhibit a superimposable reactivity on different cell lines and on platelets. Moreover, they both immunoprecipitated a 67 kDa molecule from the membrane of the reference target cells. Immunodepletion and cross-inhibition tests indicated that the two mAbs recognize two epitopes closely localized on the same molecule. The MLuC5 mAb was further characterized for its reactivity on platelets. Immunoprecipitation and ELISA assays demonstrate that this mAb recognizes the 67 kDa high affinity laminin receptor. MLuC5 reactivity was evaluated by immunohistochemistry on a variety of normal and tumor tissues, in particular breast specimens including normal epithelium, dysplastic lesions, in situ carcinomas, invasive primary carcinomas and distant metastases. The laminin receptor was found to be strongly expressed in 50% of the infiltrating carcinomas, whereas in situ carcinomas and benign lesions, as well as the normal mammary epithelium, were only weakly and focally positive. In metastatic lesions MLuC5 reactivity was only found in 11% of the samples tested, independently of the site of origin of the lesion.