HBsAg and anti-HBs neutralizing antibodies were quantitatively detected and typed by solid-phase RIA procedures in various random populations and in commercial lots of standard immunoglobulins. The prevalence of anti-HBs was considerably higher than prevalence of HBsAg, and the specificities of HBsAg and anti-HBs overlapped within the single groups of population. The results obtained provided evidence that hepatitis B virus infections are wide-spread, particularly in communities, and that the distribution of subtypes varies according to geographical patterns. The degree of cross neutralization between homotypic and heterotypic subtypes was not quantitatively constant, implying that the amounts of the single reacting determinants probably vary among HBsAg particles. It is also stressed that a number of sera containing both HBsAg and anti-HBs escape identification, since a fairly long interval can elapse during which the decreasing levels of HBsAg and the increasing levels of anti-HBs equalize in the blood.