To evaluate the possible role for receptor-based tyrosine phosphorylation in growth signaling induced by interleukin-2 (IL-2), a series of substitution tyrosine mutants of the IL-2 receptor β and γ(c) chains was prepared and analyzed. Concurrent mutation of all six of the cytoplasmic tyrosines present in the β chain markedly inhibited IL-2-induced growth signaling in both pro- B and T cell lines. Growth signaling in a pro-B cell line was substantially reconstituted when either of the two distal tyrosines (Tyr-392, Tyr-510) was selectively restored in the tyrosine-negative β mutant, whereas reconstitution of the proximal tyrosines (Tyr-338, Tyr-355, Tyr-358, Tyr- 361) did not restore this signaling function. Furthermore, at least one of the two cytoplasmic tyrosines that is required for β chain function was found to serve as a phosphate acceptor site upon induction with IL-2. Studies employing a chimeric receptor system revealed that tyrosine residues of the β chain likewise were important for growth signaling in T cells. In contrast, although the γ(c) subunit is a target for tyrosine phosphorylation in vivo, concurrent substitution of all four cytoplasmic tyrosines of this chain produced no significant effect on growth signaling by chimeric IL-2 receptors. However, deletion of either the Box 1, Box 2, or intervening (V- Box) regions of γ(c) abrogated receptor function. Therefore, tyrosine residues of β but not of γ(c) appear to play a pivotal role in regulating growth signal transduction through the IL-2 receptor, either by influencing cytoplasmic domain folding or by serving as sites for phosphorylation and subsequent association with signaling intermediates. These findings thus highlight a fundamental difference in the structural requirements for IL- 2Rβ and γ(c) in receptor-mediated signal transduction.