Monoclonal mouse antibody to streptococcal group A carbohydrate was evaluated in a diagnostic microbiology laboratory. A total of 262 isolates of beta-hemolytic streptococci were classified with commercial reagents and the monoclonal antibody to group A carbohydrate by using immunofluorescence and Lancefield precipitin tests. Both monoclonal and commercial antibodies were identically reactive by the precipitin test but significantly different by immunofluorescence. Monoclonal antibody gave the same reactivity as seen in the precipitin test, but commercial antibodies gave both false-positive and false-negative results. These results suggest that monoclonal reagents may be superior to conventional antisera.