lmmunotoxins (ITS) were prepared by covalently coupling r i c h to monoclonal antibodies (MAbs) directed against: (a) 2 different epitopes of the T-cell receptor (TcR) expressed by the Jurkat leukemia T-cell line UTi, and JTi, MAb), (b) 2 epitopes of the CD3 complex (SpV-T3b and I ID8 MAb), (c) the CD2 and the CD8 cell-surface molecules. Conjugates were assayed for their cytotoxic activity by pre-incubating the Jurkat cell line with different concentrations (llL250 ng/ml) of each IT for 2 hr at 37' C in the presence of 0. I M lactose. After washing, cells were cultured for 24 hr and their protein synthesis and proliferative capacities were assessed. Doseresponse experiments indicated that JTi ,, )Ti, and anti-CD3 (I ID8) ITS inhibited by >90% the cell line proliferation at 50 ng/ml, a 5-fold lower concentration than that required to achieve a similar effect when antLCD2 and anti-CD3 (SpVT3b) were used. After 4 hr of culture subsequent to treatment with JTi or JTi, ITS (250 ng/ml), protein synthesis was inhibited (>EOb/o). By limiting dilution analysis (LDA) we estimated that the frequency of proliferating Jurkat cells (I/ I .5) was reduced t o 1/20, IN60 and 11300 after treatment with anti-CD3 (SpVl3b), JTi, and JTi, ITS, respectively. Phenotypic analysis of 13 clones derived from JTi, IT-treated Jurkat cells showed that 50% were CD7+ CD3-JTi-variants. When bone-marrow mononuclear cells, previously mixed with low concentrations of Jurkat cells, were treated with anti-)Ti ITS, the toxic efficiency estimated by LDA was maintained whereas the growth of CFU-GM remained unaltered.