Transgenic mice carrying the human IL-2R alpha/p55 gene under the control of the SV40 enhancer/promoter were used to study the relevance of IL-2R in T-cell development. Serological analysis of the mouse lines obtained indicated transient, regulated expression of the human p55 gene, mainly confined to the early thymus, but which was also detected in lower amounts in the spleen. These mice showed degenerated thymuses, with an increased number of Thy1.2- double-negative precursor cells; they also had specific depletion of double-positive thymocytes. Transgene expression led to an increased number of intermediate-affinity IL-2 receptors (possibly ascribed to deregulated expression of IL-2R beta/p75) in transgenic thymocytes older than 12 weeks. These results suggest the occurrence of strong linkage between the IL-2/IL-2R system elements and thymic differentiation/maturation; they lend support to the idea of functionality of IL-2R expressed transiently in early stages of T-cell development.