The incidence of deficiency of the enzyme pyruvate kinase (PK) in Chinese infants was determined. Both the standard assay of erythrocyte pyruvate kinase enzyme activity and a fluorescent screening test (standardized) were used. The results of these two tests were compared. Of 1,159 infant cord blood samples studied, 26 (or 2.2%) had abnormally low levels of PK activity using the screening test, as did 24 of the samples tested by the enzyme assay. The results indicate that the frequency of a defective PK gene in the population of Guangzhou is significantly lower (p less than .05) than the previously reported defective gene frequency of 3.4% in Hong Kong. They also demonstrate the critical importance of standardization of the screening test, if most accurate estimates of gene frequency are to be derived by using this test.