We determined the titer of neutralizing antibody to hepatitis A virus in five lots of immune serum globulin and in the sera of human recipients of immune globulin using a new and sensitive procedure, the radioimmunofocus inhibition assay. The neutralizing antibody titer of four immune globulin lots ranged from 1:170,000 to 1:406,000, and was approximately 100-fold the antibody titer determined by radioimmunoassay. The neutralizing antibody titer of a Bureau of Biologics reference immunoglobulin was 1:794,000. Sera collected from 18 healthy men who had undergone prophylaxis with immune globulin (0.02 ml/kg body wt) were negative when tested by radioimmunoassay. However, 2 of 18 sera collected before administration of immune globulin and sera from all 18 men collected 3 and 55 days later contained detectable neutralizing antibody. The measurement of neutralizing antibody should prove useful for standardizing passive immunoprophylaxis against hepatitis A as well as for evaluating the potential efficacy of new hepatitis A virus vaccines.