There are at least two forms of cellular receptors for interleukin 2 (IL-2); one with a very high affinity and the other with a lower affinity. We identified a non-Tac IL-2 binding peptide with a relative molecular weight of 75,000 (p75). Cell lines bearing either the p55 Tac or the p75 peptide alone manifested low-affinity IL-2 binding, whereas a cell line bearing both peptides manifested both high- and low-affinity receptors. After the internalization of labeled IL-2 through high-affinity receptors, the p75 peptide could not be detected by cross-linking studies. Furthermore, fusion of cell membranes from low-affinity IL-2 binding cell lines bearing the Tac peptide alone with membranes from a cell line bearing the p75 peptide alone generated hybrid membranes bearing high-affinity receptors. These results suggest a multichain model for the high-affinity IL-2 receptor in which high-affinity receptors would be expressed when both Tac and p75 IL-2 binding peptides are present and associated in a receptor complex.