We have studied activation-induced changes in intracellular calcium [Ca2+]i, interleukin-2 (IL-2) secretion, and clonal abortion of the human leukaemic T-cell line Jurkat and three T-cell receptor (TcR)/CD3 receptor negative clones deficient for the TcR alpha, TcR beta and CD3 gamma chains respectively, as well as three transfectant clones reconstituted with the appropriate TcR/CD3 cDNA. For activation, the cells were exposed to anti-TcR/CD3, anti-CD2 and anti-major histocompatibility complex (anti-MHC) class I monoclonal antibodies (mAb) respectively. Cellular activation by these mAb leading to an increased IL-2 secretion was preceded by a rise in [Ca2+]i and was relatively dependent on the expression of the a TcR/CD3 complex. In contrast, anti-MHC class I mAb-induced clonal abortion in Jurkat T cells may occur without previous fluctuations in [Ca2+]i and appeared to be independent of TcR/CD3 expression. The present observation suggest the existence of different secondary messenger systems operating in Jurkat cells following activation via the TcR/CD3, CD2 and the MHC class I pathways, respectively.