In 784 DT and 15,752 DTP immunizations given to children 0 to 6 years of age who were prospectively studied for reactions occurring within 48 hours following immunization, minor reactions were significantly more frequent following DTP vaccine. The ratio of reaction rates associated with DTP and DT immunizations (DTP/DT) for selected local and systemic reactions was as follows: local redness, 37.4%/7.6%; local swelling, 40.7%/7.6%; pain, 50.9%/9.9%; fever, 31.5%/14.9%; drowsiness, 31.5%/14.9%; fretfulness, 53.4%/22.6%; vomiting, 6.2%/2.6%; anorexia, 20.9%/7.0% and persistent crying, 3.1%/0.7%. Following DTP immunization nine children developed convulsions and nine developed hypotonic hyporesponsive episodes. No sequelae were detected following these reactions.