In Triton X-100 solubilized leukocytes of 17 patients and 8 obligate carriers of X-linked recessive ichthyosis (XLI) the activity of arylsulphatase C (ASC) was determined and expressed as the ratio to beta-galactosidase activity. The ASC/beta-gal ratio of XLI patients is markedly decreased (range 0.07-0.48) in comparison to the corresponding control group of males (range 1.3-2.7). The enzyme ratios of 8 obligate carriers of XLI are decreased (range 0.90-1.9) in comparison to the normal females (2.13-5.52). These results indicate that the determination of the enzyme ratio of ASC/beta-gal in Triton X-100 solubilized leukocytes is a sensitive test for biochemical identification of patients and probably of carriers of XLI.