In Fabry disease, as in other X-linked traits, identification of all heterozygotes is difficult. Reduced plasma alpha-galactosidase activities will correctly identify 60-70% of the carriers. The identification rate improves when an alpha/beta-galactosidase activity enzyme ratio is used. We measured alpha-galactosidase activity in reference to several other enzyme activities, beta-galactosidase, beta-hexosaminidase, and alpha-fucosidase in plasma and leukocytes from 22 suspected and 9 obligate carriers from 4 kindreds of Fabry disease patients. Utilizing such ratios or various combinations of ratios in plasma we have correctly identified the carrier state in 91% of heterozygotes. Leukocyte alpha/beta-galactosidase identified one more female than leukocyte alpha-galactosidase activities alone. We recommend the use of such multiple biochemical tests to identify carriers of Fabry disease.