A radioimmunometric antibody-binding assay was developed with the use of 125I-labeled protein A of Staphylococcus aureus (SpA) for the evaluation of xenoantisera to human melanoma-associated antigens. Antisera were produced in New Zealand male albino rabbits by the injection of cultured human melanoma cells or soluble, partially purified melanoma-associated antigens isolated from these cells. Xenoantisera were rendered operationally specific for melanoma-associated antigens by absorption with human red cells and cultured lymphoblasts. The methodologic parameters and the quantitative relationships among xenoantisera, cultured melanoma target cells, and 125I-labeled SpA and their effect on the measurement of xenoantibody binding were critically evaluated. Data indicated the usefulness of the radioimmunometric assay in monitoring the efficacy of absorption and in characterizing the specificity of xenoantisera to melanoma-associated antigens. The radioimmunometric binding assay when modified and used as a binding inhibition assay was effective in the assessment of the serologic activity of soluble melanoma-associated antigens and thus may be used to monitor the progress of antigen purification.