Induction of cytolytic activity of peripheral blood mononuclear cells towards target cells was studied by preparing bispecific F(ab')2 which was composed of two Fab' fragments, one of which was derived from anti-CD3 monoclonal antibody and the other from anti-tumor monoclonal antibody. After reduction of the interchange disulfide bonds of these fragments by dithiothreitol, a thiol-disulfide interchain reagent, 5,5'-dithiobis-2-nitrobenzoic acid, was added to convert the free SH groups of one of the Fab' fragments to mixed disulfide derivatives. These were then coupled with the other Fab' fragment bearing free SH groups, producing a bispecific hybrid F(ab')2 monomer of high yield. The bispecific F(ab')2 monomers were able to render nonactivated peripheral blood mononuclear cells cytotoxic against natural killer-resistant tumor cell lines at doses as low as 1 microgram/ml. The polymeric forms of the F(ab')2 fragments prepared by use of cross-linking reagents such as N-succinimidyl-3-(2-pyridyldithiol)-propionate (SPDP) or S-acetylmercaptosuccinic acid anhydride (SAMSA) were less efficient for induction of cytolytic activity than the monomeric one. It may be feasible to use bispecific F(ab')2 monomers in cancer immunotherapy because of the ease of preparation, as well as the efficiency in inducing cytolytic activity and their high tissue permeability due to their small size (100-110 kDa). In addition, redirected cytolytic activity towards peripheral blood mononuclear cells by reticuloendothelial system cells, resulting from linking these two types of cells through Fc-Fc receptor interactions, does not occur.