SummarySome practical aspects of simultaneous immunization of infants against diphtheria, tetanus, and pertussis by means of mixed (triple) vaccines were investigated. Three injections were made, at 3, 41/2and 6 months of age. Mild febrile reactions occurred in 22 per cent of the infants. Local reactions were common, particularly when a vaccine with a stronger pertussis component was employed. Three different sites of inoculation (back, arm, and leg) were compared; preference is given to the supraspinous fossa since persistent infiltration with consequent risk of abscess was less common there. The antibody response to diphtheria and tetanus was very satisfactory. In regard to pertussis, vaccine I (45 billion pertussis bacilli per 3 cc) gave rather low agglutinin titres, whereas vaccine II (60–75 billions per 3 cc) evoked a more satisfactory respons