Responding mechanism of regenerating hepatocytes induced by partial hepatectomy was studied. 1) Fibroblast inhibitory and stimulatory factors on fibroblast proliferation co-exist in the same rat liver supernatant, and they were simply isolated each other by ethanol fractionation. 2) These factors have protein components and lose the biological activities by heat-treatment, and they show the competitive activity each other, respectively. 3) In regenerating liver, stimulatory factor(s) was not remarkably changed, but inhibitory one almost lost the inhibitory activity. These phenomena are very rational and suitable for hepatocyte regeneration. 4) The inhibitory activity was separatedly eluted by DEAE-cellulose chromatography of inactive regenerating liver fraction, and new peak that did not exist in normal inhibitory fraction was detected in polyacrylamid gel electrophoresis of regenerating liver same fraction. 5) These phenomena suggest that a possible new anti-inhibitory factor(s) would be formed by partial hepatectomy.