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Ricin A chain was coupled to murine monoclonal antibodies MBr1 and MOv2 respectively raised against human breast and ovarian carcinomas. Inhibition of protein synthesis only occurred in those cultured human tumor cells bearing the appropriate target antigens, demonstrating that both components of the conjugate were unchanged in regards to specificity and toxicity. Conjugates were 125-200 times more efficient in inhibiting [3H]proline incorporation than the uncoupled ricin A chain. They were however unable to kill the entire population of the appropriate cells even after repeated treatment. Although the two monoclonal antibodies had similar binding kinetics, the conjugates differed in their cytotoxicity kinetics. The MBr1-ricin A chain conjugate had slow kinetics, and about 20 hours were needed to obtain a protein synthesis inhibition above 50% on the appropriate line (mammary carcinoma MCF-7). In contrast, the MOv2-ricin A chain conjugate showed very fast kinetics, reaching 50% inhibition after only 30 minutes of treatment on both appropriate cell lines SW626 and HT-29 from ovarian and colon carcinomas, respectively. Growth conditions of cell lines, i.e., adherent cells versus suspended cells, and plating time were found to greatly influence the conjugates' killing efficiencies. These studies confirm the possibility of preparing ricin A chain-antibody conjugates, which retain specific cytotoxicity against tumor cells; but they also underline the need for further in vitro studies of various parameters before one considers a therapeutic use of such conjugates.