A phase 1 smallpox vaccine trial involving 350 adult volunteers was conducted. Of these subjects, 250 were naive to vaccinia virus vaccine (i.e., "vaccinia naive"). Volunteers received a new cell-cultured smallpox vaccine or a live vaccinia virus vaccine. Nine self-limiting rashes (3.6%) were observed in the vaccinia-naive group. None of the vaccinia-experienced patients had a rash. Rashes appeared 6-19 days after vaccination and had 5 different clinical presentations. Five volunteers had urticarial rashes that resolved within 4-15 days, 1 had an exanthem that lasted 20 days, and 1 each presented with folliculitis, contact dermatitis, and erythematous papules found only on the hands and fingers. Volunteers reported pruritus, tingling, and occasional headaches. Relief was obtained with antihistamine and acetaminophen therapy. No volunteer experienced fever or significant discomfort.