Friend erythroleukemia cells induced to differentiate with DMSO showed a 6-fold increase in the level of transferrin receptors with a commensurate increase in iron uptake Uninduced cells nevertheless had low levels of transferrin binding sites which showed saturability, specificity and a quantitative correlation with iron uptake, properties associated with specific transferrin receptors. Induced Friend erythroleukemia cells had more transferrin receptors than mouse reticulocytes; 440 000/cell compared with 86 000/cell. However, the rate of iron uptake per receptor was two-fold greater in mouse reticulocytes. Receptor levels in induced Friend cells decreased after prolonged culture in DMSO. The internalisation of transferrin which had previously been shown to occur with reticulocytes was also demonstrated with Friend cells using gold-labelled transferrin.