The accumulation of 59Fe and 239Pu was studied in rat hepatocytes in primary culture and in human hepatoma cells (Hep-G2 cells) and was compared with the uptake in isolated perfused rat liver and in rat liver in vivo. With respect to iron uptake from citrate both cell types react similarly: time and concentration dependence as well as the influence of temperature point to a non-specific but energy-dependent uptake mechanism. Pu shows similar behaviour except that a relatively large fraction remains bound to the cell membrane and uptake is lower. This is much more pronounced in Hep-G2 cells than in hepatocytes. If the metals are bound to transferrin, uptake into both cell lines, as well as into isolated perfused rat liver, is very low. After intravenous injection of 239Pu-citrate, accumulation in the rat liver is very rapid during the first 10 min and much slower after that. The role of citrate in metal uptake is discussed.