Monoclonal antibody production rate per hybridoma cell was measured as much larger in mouse peritoneal cavity than in vitro culture. To identify factors enhancing antibody productivity per cell, a hybridoma was cultured in vitro with ascites and peritoneal exudate cells (PEC). Both the ascites and the PEC improved the productivity per cell in vitro. Each chromatograph-fraction of ascites was assayed for the enhancing activity.