Cyclosporine A (CsA) has been reported to be able to promote cell proliferation, although the precise mechanism by which CsA stimulates cell growth remains uncertain. In the present study, we examined, in hepatocyte cultures, the effect of CsA on parameters related to the cell cycle as well as the levels of proteins involved in the control and progression of the cycle. Flow cytometry analysis detected an increase in the percentage of cells involved in the S phase of the cycle, which correlated with increases in the levels of cyclins D1 and E (two G1-progression regulators), as well as in those of PCNA (proliferating cell nuclear antigen), and without modification in p27, an inhibitory protein of CDKs. We also examined in nucleus the levels of nuclear factor kappaB (a nuclear factor involved in the transcription of the cyclin D1 gene) and found that this transcription factor increased in the presence of CsA. We conclude that the increases in cyclin D1, PCNA, and cyclin E, together with the invariable level of p27, clearly show that CsA induces hepatocytes to proliferate. These results reinforce the idea of the growth-promoting effect of CsA in cultured hepatocytes.