The effect of early immunization, prior to discharge from the newborn nursery, on subsequent immunity as determined by enzyme-linked immunosorbent assay (ELISA) immunoglobulin (Ig) M and IgG antibody titers to filamentous hemagglutinin and lymphocytosis-promoting toxin (LPT) of Bordetella pertussis and by standard pertussis agglutinin titers was investigated. Eighteen infants received routine diphtheria-tetanus-pertussis (DTP) immunization at 2, 4, and 6 months of age; 17 other infants received routine immunization and an additional DTP immunization in the newborn nursery. Antibody was determined on samples of cord blood and whole blood obtained at 4, 6, and 9 months of age. IgM anti-filamentus hemagglutinin was significantly higher at 4 and 6 months of age in the group that received early immunization (P less than .05). There was no significant difference in IgM anti-LPT, IgG anti-filamentus hemagglutinin, IgG anti-LPT, or pertussis agglutinin antibodies. Six control infants had high cord IgG anti-LPT titers. These six infants had significantly lower antibody titers to LPT at 6 and 9 months of age when compared with control with control infants with lower cord titers. Thirteen infants in the early immunization group with lower cord IgG anti-LPT titers achieved significantly lower titers at 9 months of age than the 12 comparable infants in the control group.