The influence of centrifugation on the infectivity of Chlamydia pneumoniae IOL-207 was compared with seven serovars of C. trachomatis biovar trachoma, the three serovars of C. trachomatis biovar lymphogranuloma and four C. psittaci strains. Prolonged centrifugation from 1 to 4 hours resulted in a 3-fold increase in the infectivity of C. pneumoniae and variable or no increase in the infectivity of the other Chlamydia strains studied. These findings indicate that a 4 hour centrifugation of C. pneumoniae is recommended to improve the isolation procedures of this organism in cell cultures.