The strain to be used as antigen is grown on 0.2% phosphate broth for 3-4 generations, being transplanted twice a day. On the day of the test, approximately 40 cc. of 0.2%, phosphate broth is poured into 100 cc. bottles containing sterile potato strips. The broth is then heavily seeded, 2 cc. of culture being used for each bottle. They are then incubated at 37° C. for not longer than 2 hrs., being shaken every 15 min. to stimulate growth; the 2-hour growth, however, is too thin to be used directly in the test. To obtain a proper emulsion, the culture is centrifuged 30 min. at high speed, to throw down the cells. The supernatant fluid is then poured off and the cells are resuspended in enough of the same "supernatant" to give it a greater than desired cloudiness, for this is again centrifuged for 5 min. at low speed to throw down any course granules. An antigen of desired cloudiness is thus obtained. This simple, quick and effective method of preparaing antigen gives hope of extensive diagnostic application of the agglutination test to hemolytic streptococci.