Histochemical investigation shows that ATPase activity is located intensively on the surface of cell contacts in hepatoma cells grown in confluent monolayer culture. Dibutyryl cyclic AMP and theophylline-treated hepatoma cells which exhibit contact-inhibited growth show the absence of localization of intense ATPase activity at cell-cell contacts. However, after removal of these additives the activity fully recovers to the intense level of control cells, when the release of cells from contact inhibition of growth occurs. Cultured hepatic parenchymal cells in monolayer have little or no ATPase activity at their surface immediately after contacts are established, and again after growth to a confluent state. In a different type of hepatoma cell which is less malignant and forms a small tissue mass or island, cell surface ATPase activity at cell-cell contacts in an island is very weak or scarcely detected even when cells are not treated with dibutyryl cyclic AMP and theophylline.