We investigated the rates of local and systemic reactions following 9920 diphtheria-tetanus toxoids-pertussis immunizations from 25 lots of commercially available, United States-licensed diphtheria-tetanus toxoids-pertussis adsorbed vaccines from four manufacturers as a function of vaccine lot, endotoxin content, pertussis vaccine potency and percent of mouse weight gain. There were significant differences between the rates of reactions by lot for all local and systemic reactions except convulsions and hypotonic hyporesponsive episodes. For these latter reactions there were insufficient cases for analyses. P was less than 0.0001 for local reactions, fever, drowsiness, fretfulness, anorexia and screaming and 0.017 for vomiting. No single lot was associated with the highest or lowest rate of reactions for more than 3 of the 11 reactions. There was a significant positive association of endotoxin unit (EU) content and the percent of vaccine recipients who developed fever (P = 0.004). Fever increased in frequency from 20.6% of children immunized with vaccine lots that contained 2500 EU to 55.1% of children immunized with vaccine lots containing 40,000 EU. There were significant positive associations of all local reactions and pertussis vaccine potency (P = 0.0004), and percent of mouse weight gain (P less than 0.0001). There was also a positive association of percent mouse weight gain and persistent screaming (P = 0.001). However, for the majority of reactions there was no clinically meaningful associations between reaction rates and the biological properties of the vaccine studied.