We studied the effect of co-culture with endothelial and/or Kupffer cells on hepatocyte replication in primary culture by means of [H-3]-thymidine incorporation. Hepatocytes, sinusoidal endothelial and Kupffer cells were extracted from rat liver by a two-step perfusion and subsequent two-step elutriation method. The replication was promoted in proportion with the densities of endothelial cells, but inversely suppressed by co-culture with Kupffer cells. The replicative effect of endothelial cells was not observed with the high number of hepatocytes cultured, however. The hepatocyte replication was also regulated in accordance with the ratios of endothelial/Kupffer cells. These results indicate that endothelial cells have a positive effect and Kupffer cells have a negative effect on hepatocyte replication, and that this replication may be associated with hepatocarcinogenesis and/or tumor-growth of hepatocellular carcinoma.