Bispecific F(ab'γ)_2 antibody derivatives, constructed by the disulphide or thioether linking of two Fab'γ fragments, have been used to link target and effector cells via specific surface molecules. Effects on both target and effector cells were studied, in particular the induction of antigenic modulation, changes in intracellular calcium ion concentration ([Ca²⁺]ᵢ), and apoptosis. Antigenic modulation, induced by bivalent antibody in the absence of cellular interactions, was observed with surface Ig (sIg) on tumour target cells, but not Fcγ receptor (FcγR)I, FcγRII, or FcγRIII on effector cells. Bispecific antibody derivatives (functionally univalent for individual cells) did not induce significant antigenic modulation upon interaction with only one cell type. Likewise interactions between target and effector cells, induced by F(ab'γ)₂ specific for sIg and an FcγR, yielded little modulation of either molecule. However after cell contact was ended by reduction of the disulphide bond between the two Fab'γ, significant internalisation of the now freed sIg and FcγR was seen. Increases in [Ca²⁺]ᵢ were observed upon bivalent or multivalent cross-linking by conventional antibody of sIg, CD37, or CD19 on tumour target cells, CD3 or CD2 on T lymphocytes, and all FcγR on monocytes, granulocytes, and NK cells. Binding of target cells to effector cells, by bispecific antibodies linking pairs of the above molecules, induced similar increases in [Ca^2+ ]_i, except for the absence of any signal via the granulocyte FcγRIII. Significant target cytotoxicity occurred when bispecific antibodies recruited NK cells or monocytes, but not granulocytes, via FcγR. Chelation of intracellular Ca^2+ in the NK cells or monocytes reduced the observed cytotoxicity, suggesting a role for Ca^2+ in activation for lysis. Apoptosis was shown to occur apparently spontaneously in vitro in one target cell studied, guinea-pig leukaemic lymphocytes, and was seen in NK lysis of susceptible targets. However, cellular cytotoxicity dependent upon antibody - ADCC recruited via either Fcγ of IgG or bispecificity of F(ab'γ)_2 - failed to induce apoptosis.