Whooping cough specific surface IgA antibodies, agglutinating serum IgG antibodies and the in vitro lymphocyte reactivity to Bordetella pertussis germs were investigated in newborns and infants both unvaccinated and parenterally and orally immunized against whooping cough. Furthermore the E-rosette formation and the lymphocyte reactivity to phytohaemagglutinin, pokeweed mitogen and concanavalin A were studied. Only oral pertussis immunisation effected a local immune reaction with formation of secretory surface IgA antibodies in the saliva and prevented the postnatal disappearance of the Bordetella pertussis specific IgG antibodies seen in unvaccinated infants. From the third month of life the lymphocyte reactivity to a Bordetella pertussis germ suspension resulted in measurable stimulation following oral whooping cough vaccination. After the third month of life the pertussis stimulation indices of the orally immunized infants remained above the values for adults with a history of whooping cough and were equivalent to those for the parenterally immunized infants.