Recent studies in mice harboring a targeted disruption of genes encoding TNF receptor 1 (TNFR-1) or Interleukin 6 (IL-6) suggested a critical role for TNF and IL-6 in initiation of liver regeneration after 2/3 partial hepatectomy. However, hepatocyte proliferation can also occur following treatment with agents that do not induce tissue loss (primary mitogens). To determine whether the above cytokines could also be involved in mitogen-induced liver cell proliferation, we studied the hepatocyte proliferative response after treatment with primary mitogens in mice knock-out for TNFR-1 or IL-6. Our results showed no difference in the proliferative response of the liver between the wild type and the knock-out mice following treatment with the mitogens 1, 4-bis [2-(3, 5-dichloropyridyloxy)] benzene (TCPOBOP), or the peroxisome proliferator, ciprofibrate, suggesting that TNF or IL-6 may not play a major role in this type of proliferation. Gel shift assay indicated that TCPOBOP-induced hepatocyte proliferation is not associated with activation of STAT3 transcription factor, a major target of IL-6 and other growth factors/cytokines. Our results thus indicate that hepatocyte proliferation can be induced by at least two different pathways; compensatory regeneration being TNF and IL-6-dependent, and mitogen-induced direct hyperplasia which does not require TNF or IL-6.