The ability of the Streptozyme test to identify significant antibody rises in 46 patients with streptococcal pharyngitis was comparable to, but no greater than, that of the antistreptolysin O or antideoxyribonuclease B test and inferior to that of the combined use of both the antistreptolysin O and antideoxyribonuclease B tests. Serum specimens were also simultaneously analyzed with three different lots of Streptozyme reagent. Lot-to-lot variation in the reagent resulted in a significant difference in antibody titer for 18 (19%) of the 92 sera tested. Differences among the three lots also produced variation in determining whether a significant rise in titer had occurred from the acute phase to the convalescent phase serum for a given patient. These observations raise concerns about the standardization of the Streptozyme reagent and document the need for precise identification and quantitation of the streptococcal antigens used in this product.