An immunotoxin, SOKT1-RTA, was prepared successfully by conjugating a murine anti-human T lymphocyte monoclonal antibody (McAb) SOKT1 and the ricin toxin A chain (RTA) with the heterobifunctional linker SPDP. At the presence of 20 mmol/L NH4Cl, the SOKT1-RTA conjugate killed 82% of the human peripheral blood T cells (PBT), and decreased their 3H-Leu incorporation to 6.2% of the untreated controls. The toxic effect of the immunotoxin can be partially blocked by unconjugated SOKT1, but not by an irrelevant McAb D51. These results indicated that the SOKT1-RTA conjugate still retains both functions of its component parts:binding to human PBT specifically, and inhibiting protein synthesis. It may be an effective reagent for in vitro depletion of immunocompetent T cells from human bone marrows.