The ability of haemolytic streptococci to produce NAD(+)-glycohydrolase was investigated by a fluorescent assay. Enzyme production was found in 31 (91%) of 34 group A, 17 (61%) of 28 group C and eight (27%) of 30 group G isolates. The high producers were found in 22 (65%) of group A, one (4%) of group C and none of group G isolates. The high producers of the group A isolates belonged to T-1, T-3, T-4 or T-12 serotype. These results suggest that NAD(+)-glycohydrolase productivity of streptococci is closely related to specific Lancefield's groups or T serotypes.