Rat liver cells prepared by collagenase treatment and separated by density gradient centrifugation with Ficoll-Hypaque solution (specific gravity 1.120), containing only hepatocytes (80%) and Kupffer cells (20%), were found to stimulate strongly Ag-B-incompatible lymphocytes without 2-mercapto-ethanol in culture. The same liver cells could also stimulate Ag-B-compatible lymphocytes in the presence of 2-mercapto-ethanol. A positive response was only seen when the liver cell numbers were 1∼ 2% of the responding lymphocytes. Viable liver cells inhibited the reaction in a two-way culture.