In the perspective of therapeutic approaches the monoclonal antibody, MBrl, with a quite restricted spectrum of reactivity for human breast carcinoma, was coupled to restrictocin (Res), a ribosome inactivating protein produced by Aspergillus restrictus. In a cell-free system this toxin was found to have an activity comparable to that of other plant toxins, but its in vitro toxicity was shown to be low on different cell lines. Three batches of MBr1-Res conjugate were prepared and their specificity, efficiency, and maximum level of cytotoxicity were analyzed on the cell line MCF-7 expressing the relevant antigen, on several irrelevant tumor cell lines, and on normal cells. Conjugates were from 600 to 1500 times more efficient than the uncoupled derivatized Res towards MCF-7 cells and were completely ineffective on the other target cells. The antigen-driven cytotoxicity was confirmed by the nontoxicity of an irrelevant conjugate on MCF-7 cells. The cytotoxic efficiency of MBr1-Res was low when compared to the binding level of MBr1 at the same concentration and a portion of treated cells (from 10% to 30%) survived the treatment. The heterogeneity of expression of the relevant antigen, together with its only partial internalization, could account for these limitations. The lysosomotropic agent ammonium chloride and the carboxylic ionophore monensin were tested as potentiating agents but in both cases the cytotoxicity remained unmodified. A neutralization assay performed on a xenogenic model indicated that the MBr1-Res conjugate was capable of reducing the tumor take. These data indicate the possibility of using the Res to prepare a reproducible and highly selective breast cancer conjugate. However, there are still a number of problems which must first be solved before we can consider its clinical application.