Thymocytes from severe combined immune deficient (SCID) mice undergo developmental arrest at an early stage, before most TCR gene rearrangement. They therefore represent a natural test case to assess those aspects of T cell development that are TCR independent. Multiparameter flow cytometry was used to analyze the array of immature phenotypes present in the SCID thymus at steady state, as defined by the markers CD4, CD5, Sca-1, NK1.1, CD44, heat-stable antigen (HSA), and IL-2R alpha. The results suggest a simple developmental block in SCID mice rather than a program of aberrant differentiation. SCID thymocytes displayed efficient, developmentally regulated functional responses. Approximately 20-25% of the cells, mostly within the IL-2R alpha +HSA+CD44low fraction, could be induced to express IL-2. This IL-2 inducibility was highly dependent on IL-1 costimulation, in agreement with the behavior of normal immature thymocytes. These results formally demonstrate that competence to express IL-2 is developed independently of TCR expression or gene rearrangement. Comparison of the response properties of various SCID thymocyte subsets indicated that IL-2 inducibility is first likely to be acquired at an early (Sca-1++CD44++HSAlow) stage. A later functional transition was revealed by comparing patterns of IL-2R alpha regulation in normal and SCID IL-2R alpha +HSA+CD44low thymocytes. The SCID thymocytes uniformly maintained IL-2R alpha expression on in vitro stimulation, whereas only a minority of the normal cells in the corresponding subset could do so unless IL-1 was also added. The SCID arrest point thus appears to separate the IL-2R alpha +HSA+CD44low stage into distinct early (TCR independent) and late phases. Normal cells that progress beyond the SCID arrest point appear to lose, rather than gain, competence to make various responses, even before they leave the IL-2R alpha +HSA+CD44low stage. A model is proposed in which discrete changes in functional competence define novel transitions in early thymocyte development, at least some of which may be linked to TCR-beta gene rearrangement before positive or negative selection.