The present disclosure described an assay for screening monoclonal antibodies for their potential as highly cytotoxic immunotoxins. The assay involves treating cells with dilutions of the test antibody followed by a Fab fragment of a secondary antibody coupled to an A chain toxin ("indirect assay"). The cytotoxicity of the indirect assay is compared to that of the direct assay where the monoclonal antibody is coupled to an A chain toxin. Indirect and direct assays were carried out using 14 antibodies and a panel of 8 human and mouse cell types. The two assays showed virtually 100% correlation. The indirect assay, therefore, predicts the potency of a given monoclonal antibody to make an effective immunotoxin and should be useful in screening monoclonal antibodies for use as immunotoxins.