AbstractIn an attempt to elucidate the intricate structure and possibly the signal transduction pathway of the interleukin-2 receptor (IL-2R) we tried to produce monoclonal antibodies against putative human IL-2R-associated molecules which precipitated from YT2C2 cells with a monoclonal antibody (Mikβ3) against the human IL-2Rβchain. One of the antibodies obtained (7xA10) recognizes a cell surface molecule of about 120 kDa (p120). Cross-linking of [125I]-IL-2 and immunoprecipitation with 7xA10 suggest that the p120 protein may somehow associate physically with the IL-2Rβchain. Although p120 itself did not bind IL-2, and 7xA10 did not inhibit IL-2 binding to the IL-2R, growth of the human IL-2-dependent cell lines ED40515 and Kit225 was completely inhibited by adding 7xA10 to the culture at a relatively low concentration (0.2g/ml). At a higher concentration (2g/ml) 7xA10 inhibited the growth of YTC3 and YT2C2 cells expressing IL-2Rs with high and intermediate affinities, respectively. The B cell line FLEB-14 and the erythroid cell line K562, which both express p120 as determined by flow cytometry, but no IL-2Rαorβchains showed only slight proliferation changes at high 7xA10 concentrations. Out of 16 lymphocyte cell lines tested so far, only Molt-4, Raji and Daudi did not express p120, and therefore 7xA10 did not influence their growth behavior. p120 might be a new IL-2R association molecule which seems to play a certain role in the growth regulation of various lymphoid cell lines.