Abstract: Assay methods and test kits for detection of antibodies having specificities for a variety of determinants in patient samples are disclosed. The assay is especially suitable for use in evaluating patients for their suitability to receive diagnostic and therapeutic immunoconjugates comprising immunoglobulin constituents derived, at least in part, from a non-human source. The assay methods and test kits utilize a target substance (e.g., immunoglobulin) which has substantially the same binding characteristics as a portion of the immunoconjugate proposed for in vivo administration. A plurality of target substance densities are used for assaying each patient specimen to provide an indication of both the level and relative affinity of circulating antibody capable of binding to the immunoconjugate proposed for treatment.