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The A-chain of a plant toxin ricin has been coupled to poly- and monoclonal antibodies specific to the L-chains of human IgG. The inhibitory effect of the conjugates has been compared with the ability of the antibodies to bind to target cells. Cytotoxicity of the conjugates has been monitored following incorporation of 14C-leucine radioactivity into Burkitt lymphoma cells with surface Ig. The 50% inhibition of protein synthesis is observed 18 h after treatment of cells with immunotoxins, when the concentration of the conjugates with poly- and monoclonal antibodies is 1.2.10(-9) M and 0.7.10(-9) M, respectively. The data take into account that only part of the polyclonal antibodies molecules is able to react with target cells. The control conjugates containing either monoclonal antibodies that do not react with the lymphoma cells surface L-chains or nonimmune serum IgG proved to have no effect on target cells even at the level of 10(-7) M. The immunotoxins with poly- and monoclonal antibodies produce almost the same kinetics of protein synthesis inhibition, when incubated with lymphoma cells for 60 min. However, a 30 min treatment reveals a considerably higher cytotoxicity of the conjugate with monoclonal antibodies.