ABSTRACT A total of 120 pleural fluid specimens from 113 pediatric patients were tested using two rapid antigen detection assays for Streptococcus pyogenes . Results were compared to culture, Gram stain, and PCR results. Each rapid antigen assay detected 9 out of 10 (90%) PCR-positive samples, with 100% specificity. These antigen detection assays are useful to provide microbiological diagnosis of empyema caused by S. pyogenes .