As part of a study with a quadruple inactivated vaccine (diphtheria-pertussis-tetanus-polio), the serologic response to the pertussis antigen was investigated in infants at the age of routine immunization, inoculated with one of the following two regimens: either 0.5 ml vaccine at 2 and 3 1/2 months and a booster six months later, or an identical dose of vaccine given at 2, 4 and 6 months and a booster at the age of 12 months. A pertussis agglutination titer of greater than or equal to 1:10 was considered an immune response to the administration of the antigen. Two basic doses of pertussis antigen induced an immune response in about 92% of children, which was very close to that following three basic doses. A 100% seroconversion was observed in both groups one month after the booster dose, and geometric mean values were high in both regimens. At one and two years after the booster, the pertussis agglutinins were present in 100% of children of both groups, with higher geometric mean values in the group given the three basic doses regimen.