Gentamicin, at concentrations up to 500 mug/ml, showed no effect on the replication, yield, or infectivity of seven viruses or on the agents of psittacosis and meningopneumonitis when grown in mice or embryonated eggs. At the 500 mug/ml level, lymphogranuloma venereum cultures had a slight reduction in infectivity. Rickettsia akari demonstrated susceptibility at the 5 mug/ml level, whereas R. rickettsii, R. mooseri, and R. canada grown in embryonated eggs were susceptible in varying degrees to gentamicin at or above the 50 mug/ml concentration.