A clinical study was carried out in volunteer subjects to determine the proper dosage of rabies immune globulin (human) (RIGH) when used in conjunction with rabies vaccine of duck-embryo origin (DEV). Two lots of RIGH were prepared from plasma pools derived from donors with high titres of rabies-neutralizing antibody; both lots had potencies in excess of that accepted for antirabies serum of equine origin. The subjects had never received rabies vaccine but would oridinarily have been given pre-exposure rabies prophylaxis. Serological results obtained during a period of 90 days after the initiation of the study justify the following conclusions: (1) the half-life obtained with RIGH is in agreement with that estimated for homologous passive immune globulin in man; (2) readily detectable levels of rabies antibody were found in all subjects 24 hours after the administration of 40 and 20 international units per kg of RIGH, but not after the administration of 10 international units/kg; and (3) there was an indication that 40 international units/kg may have interfered with optimal active antibody production by the vaccine, and that interference was absent or minimal after doses of 20 intenational units/kg.