After mitogenic or antigenic stimulation, T cells express interleukin-2 receptors (IL-2R). The mechanism and control of signal transduction following binding of IL-2 to IL-2R are poorly understood. Using two rat monoclonal antibodies (5A2 and 7D4) specific for two distinct epitopes of the p55 subunit of mouse IL-2R, we have studied the cellular localization of this molecule by immunocytochemistry during the IL-2-mediated activation of mouse T helper cell clone HT-2. During the activation cycle, nuclear staining for the p55 subunit of the IL-2 receptor was transiently observed. It is suggested that the transient nuclear location of the IL-2R may play a critical role in the control of T-cell activation, proliferation and/or differentiation.