Thirty cases of hepatocellular carcinoma (HCC) were investigated immunocytochemically for expression of β1 integrin molecule and of collagen IV. Immunoreactivity was related to the tumour proliferation index, as detected by PCNA immunostaining, and to tumour size and grade. Membrane β1 integrin immunoreactivity was deteccted in the neoplasticc cells of all cases, though two different staining patterns were clearly recognized. In 14 cases, β1 integrin immunoreactivity was confined to the cell-stroma interface, showing the same polarized pattern as the non-neoplastic cell counterpart. This staining pattern was associated significantly (P 0.0001) with low PCNA labelling (i.e. less than 20 per cent of neoplastic cells showing nuclear immunostaining. Conversely, 16 cases showed non-polarized pericellular β1 integrin immunostaining. This staining pattern was significantly associated (P < 0.0001)) with high PCNA labelling (more than 20 per cent of immunoreactive cells) and with tumour size greater than 4 cm in diameter (P < 0.0001). β1 Integrin, collagen IV, and PCNA immunoreactivities, however, did not correlate with the histological grade. The data emphasize that neoplastic progression of HCCs may be correlated with an aberrant expression of adhesion molecules and with a disruption of the collagen IV complement of basal membranes.