In 1981 a set of 256 of the umbilical cord blood collected from mothers between 15 and 34 years of age was serologically examined for specific antibodies against B. pertussis and B. parapertussis infections. The mothers were divided into four age groups, each with primipara and multipara subgroups, with the aim to assess age-related variations in specific immunity levels acquired as a consequence of active immunization against pertussis, or by clinical or subclinical infection with B. parapertussis (no specific immunization against B. parapertussis infection is carried out in Czechoslovakia). The percentage of mothers with B. pertussis-specific antibody was found to fall from 81% and 70% in the two younger age groups (15--19, 20--24 years) to 21.3% and 16.3% in the two older age group (25--29, 30--34 years). The variations between the primipara and multipara subgroups were not statistically significant. Antibody against B. parapertussis infection was present in 38.6% of all mothers, which is suggestive of a relatively high circulation of this agent in the population. In the two older age groups (25--29 and 30--34 years) multiparous mothers were found to have B. parapertussis antibodies at significantly higher rates than primiparous mothers, which seems to confirm the theory that B. parapertussis infection may serve as booster to immunity in mothers of families with more than one child.