A colorimetric method for measuring anti-NAD+ glycohydrolase in human sera has been developed. The assay involves the inhibition of NAD+ glycohydrolase (EC 3.2.2.5) by the antibody and determination of the noninhibited enzyme activity by using an enzymatic amplifying system for NAD+. The assay is easily carried out and has the additional advantage of a direct relationship between signal and antibody concentration. The results obtained for 100 human sera compare favorably with other tests commonly used to obtain evidence of streptococcal infections or their complications, such as the anti-streptolysin O and the anti-DNase B tests.