Abstract Cell-substratum adhesion of rat hepatocytes was inhibited by antisera raised against purified plasma membranes of rat liver (anti-liver-antiserum) and Morris hepatoma 7777 (anti-hepatoma-antiserum). It is assumed that substances which block the adhesion-inhibiting activity of the antisera are involved in cell-substratum adhesion. Adhesion-involved molecules of rat liver monitored as ‘blocking activity’ were compared with those of Morris hepatoma 7777 and 9121. They were found to be integral membrane glycoproteins, which could be solubilized only by detergents. Fractionation of plasma membrane extracts by size exclusion HPLC revealed two blocking activity peaks representing molecules involved in the adhesion to plastic (P-AIM) and collagen (C-AIM). In rat liver both adhesion-involved molecules were found; yet P-AIM seemed to be the major type of adhesion-involved molecule. In the relatively well differentiated Morris hepatoma 9121 also both types were detected. In membrane extracts of the high malignant and poorly differentiated Morris hepatoma 7777, however, no P-AIM but only C-AIM were found. Estimation by size exclusion HPLC revealed molecular weights of 120 kD for C-AIM and approx. 105 kD for P-AIM. On SDS gel electrophoresis proteins in the region of 95 kD were found in C-AIM containing fractions, whereas proteins of 105 kD are likely candidates for P-AIM.