The identification of female carriers of Fabry's disease is important for genetic counselling since prenatal diagnosis of affected fetuses is possible. The activities of either total alpha-galactosidase or alpha-galactosidase A in cultured fibroblasts were similar in Fabry carriers and controls and cannot therefore be used for carrier detection. Better discrimination between carriers and controls was found when total alpha-galactosidase activity was expressed as a ratio to beta-galactosidase activity, but overlap still occurred. However, there was complete discrimination between the ratio of alpha-galactosidase A to beta-galactosidase in cultured fibroblasts from five carriers of Fabry's disease and either 11 controls, seven hemizygote affected males or two of their female relatives.