Human alloreactive T-cell clones derived from cells invading a rejected kidney allograft were found to be capable of proliferating upon specific antigenic stimulation in short term assays and in the absence of exogenous IL-2. In most culture supernatants there was no evidence of any activity capable of triggering proliferation of the murine IL-2-sensitive CTL-L2 line. The ability of these clones to produce IL-2 was investigated under several experimental conditions. A combination of phorbol ester plus calcium ionophore was very efficient in all clones tested (17/17) in inducing transcription of the IL-2 gene. Conversely, when cell activation took place through the CD3/Ti complex, we regularly failed to detect hu-IL-2 transcripts even when the number of stimulator cells was increased. However, a blockade of IL-2-R with a monoclonal antibody inhibiting ligand receptor interaction while decreasing proliferation induced by the antigen in dose-dependent fashion permitted the presence of IL-2 to be demonstrated in supernatants. These results strongly suggest that IL-2-driven proliferation is essential in this model and may represent a major, although not exclusive, pathway leading to proliferation of these clones.