In order to check the purity of HBs antigen purified by an industrial procedure, a variety of analytical methods were applied on each purification batch intended for vaccine manufacture. These techniques included: electron microscopy, counterelectrophoresis using monospecific antisera, immunoelectrophoresis, quantitative aminoacid analysis, polyacrylamide gel electrophoresis in denaturing and non denaturing conditions, RIA determination of HBs antigen. The purity of the preparations was demonstrated and differences between the ay and ad subtypes in the RIA HBsAg determination were pointed out.