In an open randomized study 17 patients with a positive culture for Bordetella pertussis were treated for 10 days with erythromycin (50 mg/kg/day divided in 2 doses). The bacterium could not be isolated during therapy and in only one patient was it isolated 5 days after cessation of treatment. In comparison B. pertussis was isolated 10 and 15 days after diagnosis from 10 and 4 patients, respectively, of a group of 21 untreated controls. The treated group developed significantly fewer whoops than did the control group, even though most of the individuals had reached the paroxysmal stage at diagnosis. The dose of erythromycin (ethylsuccinate and stearate preparations) gave serum concentrations about 100 times larger than the minimal inhibitory concentration of isolated bacteria and was well-tolerated. Thus adequate erythromycin treatment eliminates B. pertussis from the nasopharynx and reduces symptoms in patients having a history of pertussis of less than 14 days. Adequate dosage and length of treatment might be crucial for these results.