In an open-label study, we evaluated the safety and immunogenicity of a diphtheria-tetanus-pertussis (DTP) vaccine containing an acellular pertussis component. Thirty-one normal, healthy children 4 to 6 years old received a single dose. All subjects had received their primary DTP series and DTP booster at age 18 months. All but three of the subjects were followed up for one month. Two subjects developed fever (100 F [37.8 C]) during the 24 hours after dosing. Eleven subjects had mild local reactions reported by parents at the 24-hour visit. One subject had a small "sterile abscess." Paired antibody assays revealed enhancement of titers in 19 of 21 for pertussis, 20 of 21 for diphtheria, and 19 of 21 for tetanus. Although the sample is small, the experimental vaccine appears to have been safe and effective in the subjects studied.