When lymphokine activiated killer cells (LAK cell) induced by IL-2 are infused into the same strain C(3)H mice following 70% partial hepatectomy, they display an action to suppress liver regeneration. Therefore, when Cyclosporin A (CsA) which inhibits IL-2 production was administerd to mice following partial hepatectomy, we studied as our indicator the labeling index of hepatic parenchyma cells using Brdu. Our findings showed that proliferation of live cells at 36 hours following partial hepatectomy was promoted by CsA, and the streongest promotion occuring at a dose of 5mg/kg. Before anf after hepatectomy when CsA was administered at 10mg/kg for 3 dadys, a further strong proliferation increasing effect was observed, and under these conditions at 48 and 60 hours after hepatectomy a significant increase in liver weight was observed. Give the above results, CsA it thought to have a promoting effect in liver regeneration following hepatectomy.