In previous reports, the authors demonstrated that M-CSF was produced by primary-cultured non-parenchymal (NPLC) and parenchymal (PLC) liver cells. In order to clarify the biological role of M-CSF produced by the liver, macrophage colony-stimulating factor (M-CSF)-producing cells in vivo were investigated using reverse transcriptase polymerase chain reaction (RT-PCR), dot blot analysis, in situ hybridization and immunohistochemistry. M-CSF mRNA was constantly identified by RT-PCR in the liver, NPLC and PLC, before and after partial hepatectomy. Dot blot analysis showed that fluctuations of M-CSF mRNA level after partial hepatectomy were not statistically significant. In situ hybridization revealed that M-CSF mRNA was expressed mainly in NPLC and vascular endothelial cells (VEC). In addition, a small number of PLC also expressed M-CSF mRNA. Neither the distribution nor the frequency of M-CSF mRNA positive cells in regenerative livers differed significantly from normal livers. M-CSF immunoreactivity was present in NPLC and VEC at all the times before and after partial hepatectomy, while PLC exhibited M-CSF immunostaining 0.5 days after partial hepatectomy. As normal liver expressed M-CSF mRNA to the same degree as regenerative liver, hepatic M-CSF mRNA production in vivo may be related to the physiological function of the liver. However, transient expression of M-CSF protein in PLC at an early stage after partial hepatectomy may be associated with liver regeneration.