An early biochemical event associated with T cell activation through the interleukin-2 receptor (IL-2R) is tyrosine phosphorylation of several intracellular substrates. The exact mechanism by which IL-2 regulates transcription of different genes is presently unknown. Here, we report that stimulation through the IL-2R induced tyrosine phosphorylation and subsequent nuclear translocation of stat3, a newly identified member of the signal transducers and activators of transcription (STAT) family of proteins. In contrast, stat1 proteins were not tyrosine phosphorylated after IL-2 ligation, whereas tyrosine-phosphorylated stat1 proteins (91 and 84 kDa proteins) were translocated to the nucleus following interferon-gamma treatment of HeLa cells. Apart from stat3, another cytoplasmic protein was tyrosine phosphorylated and subsequently translocated to the nucleus in response to IL-2. This protein had an apparent molecular mass of 84 kDa and was not recognized by stat3 or stat1 mAb or antisera. Since IL-2 induced nuclear translocation of the 84 kDa protein and stat3 followed identical kinetics, p84 is a candidate for a new, yet undefined, member of the STAT family. Taken together, we report that IL-2 induces tyrosine phosphorylation and subsequent nuclear translocation of stat3 and an as yet undefined 84-kDa protein in antigen-specific human T cell lines.