In preparation for a large efficacy trial in Germany, a pilot study was initiated in December 1990. In this study 149 infants were enrolled; with double-blind randomization 75 received Lederle/Takeda acellular pertussis component diphtheria-tetanus-pertussis vaccine (APDT) and 74 received Lederle whole-cell pertussis component diphtheria-tetanus-pertussis vaccine (DTP). The mean age at first dose was 3.5 months, and the second and third doses followed at 6-week intervals. Reactions were relatively mild with both vaccines; in general they were less frequent following APDT. The IgG antibody responses to lymphocytosis promoting factor (LPF) and fimbriae-2 were similar in both groups whereas the responses to pertactin and filamentous haemagglutinin (FHA) were greater in APDT recipients. DTP recipients had greater responses to tetanus and diphtheria toxoids. When age of first dose was examined (8-12 weeks versus 16-20 weeks), it was found that young age had a suppressive effect on antibody responses in DTP but not APDT recipients to LPF toxoid, pertactin, fimbriae-2, and tetanus and diphtheria toxoids. High values of transplacentally acquired antibody lessened the response to LPF toxoid and tetanus toxoid in DTP recipients and to tetanus toxoid in APDT vaccinees. The IgG immune response to LPF toxoid, FHA and fimbriae-2 was found to be more uniform in APDT recipients than in DTP vaccinees. An IgA antibody response to fimbriae-2 was noted in 13% of DTP recipients but in no APDT vaccinees. The broad immunogenicity and mild reactogenicity of this APDT vaccine justifies its use in the German efficacy trial.