The binding of the 59Fe and 239Pu complexes of transferrin and 125I labelled transferrin [Tf (125I)] to isolated cell membranes of rat liver has been studied. Transferrin forms a complex with an integral protein of the membrane which has an apparent molecular weight of about 180 kDa and is stable only at pH 7.4. Iron-59 is eluted from Sephacryl S 300 columns together with Tf (125I) or the Tf-membrane protein complex while 239Pu seems to be bound to different membrane protein(s). After isolation of the Tf-binding protein from 35S-labelled membranes and incubation with one of the metal-Tf complexes 59Fe elutes from a Sephacryl S 300 column together with 35S at an apparent molecular weight of ca. 250 kDa while 239Pu is found in fractions of lower molecular weight. It is concluded from these results that there are Tf-receptors in the liver cell membrane to which iron transferrin may bind. Plutonium, however, seems to be dissociated from Tf and bound directly to other membrane proteins.