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A quantitative method for determination of opsonins for Brucella has been described (1). Active brucellosis is associated with an increase in blood opsonin, in certain cases to more than 10 million times normal (2). The principles of determination of opsonin for Brucella have now been applied to determination of opsonins for rickettsiae.The These studies disclosed that the underlying principles affecting phagocytosis of rickettsiae are identical with those observed with bacteria such as Brucella. Significant differences in opsonin titer were found between a group of people free of contact with C. burnetii and groups immunized by vaccination or by living where Q fever was endemic. During Q fever the opsonin titer may increase many million times. In certain instances the opsonin test was more than 10,000 times as sensitive as the complement fixation test. Sources of error in testing for opsonin were also disclosed. MethodIn general, the determination of opsonin for C. Imrnetii resembles that for Brucdla (2). It differs only in standardization of rickettsial suspensions and in the stain for visualizing intracellular rickettsiae. To 0.1 ml. of heparinized blood or of equal parts of washed blood cells and diluted plasma was added 0.1 ml. of a standardized rickettsial suspension. This mixture was agitated by rotation for 30 minutes at 37°C. (1). Smears on glass slides warmed to facilitate spreading were stained with Macchiavello stain after drying (1). The elementary bodies of C. burnetii stained ruby red and the nuclei blue. The percentage of polymorphonuclear neutrophils containing elementary bodies was determined by examining 50 neutrophils.Standardization of Rickettsial Suspensions.--The AD strain of C. burnetii was employed 61 on

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