Sera from individuals across the USA and Peace Corps Volunteers (PCV) were assayed for neutralizing antibody to human diploid cell rabies vaccine (HDCV). A pairwise comparison between intramuscular (i.m.) and intradermal (i.d.) administration, PCV and non-PCV, revealed statistically significant differences between i.m. and i.d. for PCV and non-PCV, and between PCV and non-PCV for i.d. and i.m. administration. Survival analysis and frequency tables employing chi 2 tests determined that the percentage probability of maintaining an adequate titre from 1.5-2.0 years after primary vaccination was 99% in non-PCV i.m., 93% in non-PCV i.d., 88% in PCV i.m., and 64% in PCV i.d.