Background It is well known that platelets have a thrombotic effect. However, platelets play an important role not only in hemostasis but also in wound healing and tissue regeneration. Platelets have been reported to accumulate in the liver and promote liver regeneration after an extended hepatectomy, but the mechanism is unclear. The present study was designed to clarify the mechanism by which platelets have a direct proliferative effect on hepatocytes in vitro. Materials and methods Hepatocytes obtained from male BALB/c mice by collagenase digestion and immortalized hepatocytes (TLR2) were used. To elucidate the mechanism of the proliferative effect of platelets, DNA synthesis of hepatocytes was measured under various conditions and the related cellular signals were analyzed. Chromatographic analysis was also performed to clarify which elements of platelets have mitogenic activity. Results DNA synthesis significantly increased in the hepatocytes cultured with platelets ( P < 0.001). However, when the platelets and hepatocytes were separated, the platelets did not have a proliferative effect. Whole disrupted platelets, the supernatant fraction, and fresh isolated platelets had a similar proliferative effect, while the membrane fraction did not. After the addition of platelets, both Akt and extracellular signal-regulated kinases ERK1/2 were activated, but extracellular signal-regulated kinase STAT3 was not activated. Some mitogenic fractions were obtained from the platelet extracts by gel exclusion chromatography; the fractions were rich in hepatocyte growth factor and IGF-1. Conclusions Direct contact between platelets and hepatocytes was necessary for the proliferative effect. The direct contact initiated signal transduction involved in growth factor activation. Hepatocyte growth factor, vascular endothelial growth factor, and insulin-like growth factor-1, rather than platelet-derived growth factor, mainly contributed to hepatocyte proliferation.