Legend. Dose response of Rickettsia rickettsii (strain Sheila Smith [SS]) to doxycycline by a plaque-reduction method [1]. Doxycycline hyclate (concentration, 850 ,ig/mg; Pfizer Laboratories, New York) was incorporated at various concentrations into an agarose overlay medium in 35-mm plastic petri dishes that contained a monolayer of chicken embryo cells from embryonated specific pathogen-free eggs (Spafas, Norwich, Conn.) that had been infected with 100-200 pfu of plaque-purified R. rickettsii (strain Sheila Smith) that had been passaged in yolk sacs (Spafas). Plaques were counted after incubation for seven days at 32 C in a moist atmosphere of 95% air and 5% CO2,; results were expressed as a percentage of the results of control determinations (no antibiotic). The curve represents the average of three separate experiments, each using six replicate plaque dishes at each drug concentration (0.01, 0.025, 0.05, 0.075, and 0.1 MAg/ml). Results at a drug concentration of 0.5 pg/ml were also determined (data not shown). Summary