The protective value and safety of human diploid cell strain rabies vaccine (HDCV) are well established [6, 8, 11]. At present, however, the high cost and small yields of HDCV has restricted its use, particularly in developing countries where rabies vaccine is needed in large amounts. The inexpensive duck embryo rabies vaccine (DEV), also prepared with the Pitman -Moore strain, is potent and generally safe despite occasional allergic complications [9, 13]. The vaccine has been used with success in several million doses during recent decades [12, 14]. Administered according to the prescribed schedule (14 initial inoculations and 3 boosters), the vaccine has been shown to provide protective antibody titers in pre- and post-exposure use [7, 10]. On the other hand, the immunogenicity of this vaccine did not shorten the administration schedule in a similar way to HDCV. Thus, prior clinical studies using the reduced WHO immunization scheme demonstrated that a concentrated and partially purified DEV containing less than 2.5 IU/ml of rabies antigen elicited sufficient neutralizing antibodies in only 75% of vaccinees [3]. Gl uck et al. [4] clarified the reason for this low immunogenicity: We demonstrated that only about 30-50% of the glycoprotein (GP) in DEV was present on the surface of virus particles. GP, a component of the rabies virus envelope, is the antigen responsible for the induction of neutralizing antibodies. During virus production a non-immunogenic soluble GP may also accumulate. Non-immunogenic GP may be coextracted during vaccine preparation.(ABSTRACT TRUNCATED AT 250 WORDS)