In a randomised phase I trial of a recombinant vaccinia virus vaccine expressing the gp160 envelope gene of the human immunodeficiency virus (HIVAC-1e) 35 healthy, HIV-seronegative males, 31 of whom had a history of smallpox immunisation and 4 of whom were vaccinia naive, were vaccinated and then boosted 8 weeks later with HIVAC-1e or standard NY strain vaccinia virus. The frequency, duration, and titre of virus isolation from the vaccination site and occurrence of local side-effects were similar between the two groups of vaccinees. Vaccinia-naive (vac-n) subjects shed virus from the vaccination site for longer and at a higher titre than did vaccinia-primed (vac-p) individuals (19 vs 7 days and 10⁷ vs 10⁵ pfu/ml, respectively). In-vitro T-cell proliferative responses to one or more HIV antigen preparations developed in 13 of 16 vaccinia-primed subjects inoculated with HIVAC-1e. T-cell responses were, however, transient and in no subject did antibodies to HIV become detectable. The 2 vaccinia-naive subjects vaccinated with HIVAC-1e showed strong T-cell responses to homologous and heterologous strains of whole virus and to recombinant gp160 protein that remained detectable for over a year; antibodies to HIV envelope also developed in both. Recombinant vaccinia virus vaccines induce T-cell priming to the foreign gene products in most individuals. If used as the sole immunising agent they will be most efficacious in vaccinia-naive individuals.