One lot of a nationally distributed diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccine was recalled in January 1999 because of a subpotent diphtheria toxoid component. To evaluate vaccine immunogenicity, children who had received the recalled lot for at least 2 of the 3 doses of their primary series were identified. Diphtheria antitoxin (DAT) levels were then prospectively assessed before and after dose 4 of (fully potent) DTaP vaccine. Of the 105 children evaluated, 84% had prevaccination DAT levels <0.10 IU/mL, which is the level generally accepted as protective. DAT levels rose a mean of 92-fold after dose 4; 100% of subjects had DAT levels >or=0.10 IU/mL, and 69% had DAT levels >or=1.0 IU/mL. These results indicate that diphtheria potency testing can identify vaccine that is less immunogenic when administered during the primary series. The booster response to dose 4, although reduced, was sufficient to confer adequate protection in the interval before receipt of the fifth dose of DTaP.