A radioimmunoassay method for subtyping hepatitus B surface antigen (HBsAg) was developed. Polystyrene beads pretreated with glutaraldehyde and coated with monospecific anti-d or anti-y antibodies were used. The treatment of beads with glutaraldehyde significantly increased the sensitivity of the test. Subtyping by radioimmunoassay was found to be more sensitive than by the immunodiffusion method. Fifty-nine percent of the samples tested could be subtyped by RIA and only 32% by immunodiffusion. The technique has the added advantage of utilising a commercially iodinated anti-HBs.