Primary vaccination by intradermal jet injection, using diluted smallpox vaccine, was compared with multiple-pressure vaccination in 625 Jamaican children. The cutaneous and antibody response patterns were evaluated. The primary take rates among those jet vaccinated were 97% or more in those receiving vaccines with a titre of 10(6.3) TCID(50)/ml and 10(7.0)TCID(50)/ml, and 96% in those vaccinated by multiple pressure, using undiluted vaccine. The primary take rates in subjects receiving jet-injected vaccine with titres of 10(6.0) TCID(50)/ml and 10(5.0) TCID(50)/ml were 90% and 62%, respectively. Among subjects tested who developed Jennerian vesicles, all but 3 demonstrated seroconversion. In those who failed to develop primary Jennerian vesicles, there was also a failure of neutralizing-antibody development. Vesicle and scar sizes were generally smaller in the jet-vaccinated subjects than in those vaccinated by the multiple-pressure technique. Infants tolerated jet vaccination without difficulty. Vaccinial complications did not occur in any subject. The intradermal jet injection of 0.1 ml of vaccine with a titre 10(6.3) TCID(50)/ml or higher is recommended as a highly effective method for achieving successful primary smallpox vaccination. The method appears best suited for use in mass smallpox-vaccination programmes.