Antibody response to streptococcal antigens was measured by three conventional methods (ASO, AH, and ANADG) and by the new streptozyme test. Compared to the others, the streptozyme test showed a sensitivity of 73% and a specificity of 90%. It also detected additional cases missed by the other methods. In addition, the streptozyme test showed antibody rise earlier, after 6 to 9 days in half of the cases. Thus, the streptozyme test is more sensitive and no less specific than any individual test and reveals antistreptococcal antibody rise earlier. These properties, together with its ease of performance and reproducibility, seem to make it the test of choice for estimation of antibody response to streptococcal infection in all circumstances where the performance of a battery of tests is impractical.