A sensitive radioimmunoassay for rabbit immunoglobulin allotypes has been developed. Separation of antibody-bound and free antigen was accomplished by salt precipitation with ammonium sulfate, followed by collection and wash on glass fiber filters by means of a semi-automatic cell harvester. The efficiency of the collection was increased by the addition of nonionic detergent which was salted out together with the precipitated reagents. The presentation of the a 1 allotype, located in the VH region, was found to be different on IgG and IgA molecules.