Using indirect immunofluorescence, the distribution pattern of fibronectin has been studied in a variety of solid human tumours and corresponding normal tissues. Fibronectin could be detected on normal epithelial cell membranes, in periglandular and perivascular basement membranes and in the intercellular stroma. No modification of this distribution pattern was detected in a variety of benign tumours or in certain pathological, non-malignant disorders (Dupuytren's contracture). In infiltrating mammary carcinomas, cell membrane staining disappeared even before invasion, the staining pattern of the glandular basement membranes became patchy, discontinuous and finally disappeared with invasion. Fibronectin persisted on vascular basement membranes and in the fibrous matrix remaining between the areas of infiltration. Similar modified fibronectin patterns were observed in bronchial, endometrial, prostatic, urinary bladder and thyroid carcinomas. In sarcomas, fibronectin disappeared from the membranes of transformed cells and decreased in intensity in the intercellular matrix. It can be stated, therefore, that the progressive loss of cell membrane and glandular basement membrane bound fibronectin is a characteristic feature of malignant human tumours and may assist with the histological differential diagnosis of atypical lesions and early invasion.