The tick hemolymph test is designed to detect rickettsial pathogens in experimentally and naturally infected adult ticks and to provide a means for rapid fluorescent identification of the antigenic group to which the pathogens belong. The test consists of obtaining hemolymph from a wound produced by amputating the distal portion of one or more legs. The hemolymph is collected on a slide, heat fixed, stained by Giménez' method, and examined microscopically. Validity of the test was established with ticks that had been infected experimentally with Rickettsia rickettsi or Rickettsia canada by feeding them as larvae or nymphs on rickettsemic rodents. Ticks with positive and negative tests were triturated separately and suspensions were injected into guinea pigs. Only ticks with positive tests produced infections typical of these pathogens. The test was successfully applied also to field ticks. In this connection it was found that Dermacentor andersoni from western Montana may harbor a rickettsia-like agent with an antigenic relation to R. rickettsi but not pathogenic for guinea pigs. The hemolymph test is an economical, rapid technique for detecting rickettsiae in adult ticks. Because its application leaves ticks undamaged, infected specimens can be used to identify rickettsial agents.