Interleukin-2 (IL-2) is a T-cell-derived polypeptide hormone of 133 amino acids which exerts its growth-promoting activity via a surface receptor¹. Originally, IL-2 was believed to be a unique growth factor for activated T cells²; more recent studies, however, have demonstrated that certain B-cell tumours³ as well as normal activated B lymphocytes^4–6 express a surface molecule which is recognized by monoclonal antibodies directed against the IL-2 receptor. Furthermore, we⁷ and others⁶ have shown recently that activated B cells proliferate in response to either immunoaffinity-purified⁸ or recombinant⁹ IL-2. These controversial findings prompted us to undertake a detailed quantitative comparison of IL-2 receptor expression on activated B and T cells. We show here, using biosynthetically labelled IL-2 (³H-IL-2) and anti-IL-2 receptor antibody (³H-PC61) that activated B and T cells express both high-affinity (apparent dissociation constant, K_d ∼ 20 pM) and low-affinity (K_d ∼1,000 pM) IL-2 receptors. Binding of IL-2 to both classes of receptor is inhibited by the monoclonal anti-IL-2 receptor antibody PC61. B blasts express half as many total IL-2 binding sites or PC61 binding sites as T blasts, and the ratio of the number of low- to high-affinity receptors for each cell type is ∼10:1. Immunoprecipitation analysis of surface-labelled blasts indicates that B and T cells have IL-2 receptors of similar relative molecular mass. Taken together, these data suggest strongly that IL-2 can act as a growth hormone for both B and T lymphocytes.