NUMEROUS studies have been reported evaluating the effectiveness of various antimicrobial agents against strains of Bordetella pertussis in vitro, and in the treatment of pertussis.1-14 Several drugs have demonstrated activity in both test situations. In most clinical series, however, significant symptomatic improvement has not been noted, and better therapeutic agents are needed. The purpose of this study is to determine the antimicrobial sensitivity patterns of recently isolated strains of B pertussis to nine antimicrobial agents, some of which have only in the past few years become available, so that data as to their activity against this organism are lacking. Materials and Methods The test organisms consisted of 36 recently isolated strains of B pertussis. Twenty-four strains were isolated from epidemiologically unrelated cases of pertussis which occurred in the New Orleans area in the spring and summer of 1967. Twelve strains which were obtained from Grand Rapids, Mich, had been