Multivalent hybrid antibodies with dual specificity were prepared by cross-linking with protein A two antibodies of different specificity, one against ricin toxin and the other against the H-2 antigens of murine leukemia EL4 cells. This bifunctional antibody specifically attached to EL4 cells was able to capture ricin toxin (RcA2) molecules with an affinity 20 times higher than that of the galactose-containing glycoproteins of the cell surface (nonspecific binding). The hybrid-bound RcA2 gained access into the target cell cytoplasm by endocytosis and blocked [3H]thymidine incorporation as efficiently as free RcA2 does on nontreated EL4 cells (3.3 X 10(-11) M). These results indicate that multivalent hybrid antibody, easy to prepare in purified form and endowed with high affinity for both target cell and ricin toxin, may be utilized efficiently for the specific delivery of toxins to target cells.