A simplified schedule, applicable in developing countries, has been tested for diphtheria, tetanus and pertussis (DTP) and polio immunization. It comprised two injections, six months apart starting at the age of 3 to 8 months. DTP and inactivated polio (IPV) vaccines of special composition and live measles vaccine given at the age of 9 to 14 months induced a good antibody response. Special attention is being given to pertussis immunity. Although low agglutination titers may sometimes be found, DTP vaccination was shown to interrupt the normal periodicity of pertussis epidemics. From the relatively high proportion of vaccines which attained pertussis specific serum IgA antibodies in the course of four years following vaccination, it could be deduced that vaccination does not prevent infection although it protects against disease. Based on these results, successful immunization against seven diseases will be possible in two sessions; DTP, IPV, and BCG vaccinations at the age of 3 to 8 months; and DTP, IPV, measles and yellow fever vaccination at the age of 9 to 14 months.