During the period 1975-1985, 35 women with serology- or culture-confirmed pertussis at the time of labor were admitted to the Danderyd Hospital (Danderyd, Sweden). In 32 cases, the mothers were allowed to nurse their newborns while receiving erythromycin therapy. Erythromycin prophylaxis was given to 28 newborns. None of the newborns developed clinical or laboratory signs of whooping cough. The therapy was well tolerated by the newborns and did not affect the microflora in the gut. Maternal antibodies to pertussis toxin and to the filamentous hemagglutinin were found in cord blood, and levels decreased during the follow-up period. The study demonstrated that mothers with pertussis can safely be allowed to nurse their infants if both receive erythromycin.