IN the course of studies on antibiotic therapy of rickettsial infection in the guinea pig, it was found that oxytetracycline, chlortetracycline and tetracycline administered intramuscularly were tolerated well in the dosages employed. The administration of erythromycin on an equivalent dose-weight basis, however, resulted in death of a portion of the test animals. The picture observed was comparable to that noted by Kaipainen and Faine1 following oral, intraperitoneal or intravenous administration of the drug. Because the response of the guinea pig to erythromycin appears to differ from a simple toxicity phenomenon, the following observations are reported.