Antibodies to diphtheria and tetanus toxoids were assayed in 46 children from 16 to 33 months of age following vaccination. A reversed rocket immunoelectrophoretic technique was used for antibody determination. This method is simple and consistent, and although the limit of detection for antibodies of both types (about 1 U/ml) is above the normal range for primary immune responses, antibody levels in immunized children five weeks after a second injection of vaccine were within measureable range in all but two of the children studied. Levels more than one standard deviation above the mean were found in one child for both antitetanus and anti-diphtheria and in four children for anti-tetanus toxoid antibody only. All four of the high responders to tetanus toxoid were Black, carrying both Black and Caucasian Gm haplotypes. The two apparent nonresponders were Caucasian. All children had normal immunoglobulin levels, except for one with low IgA, who showed normal antibody responses to both toxoids. The results suggest that measurements of precipitating antibodies to diphtheria and tetanus toxoids provide a practical index of humoral immune function in small children.