Oxyquinoline is a heterocyclic phenol and Oxyquinoline Sulfate is its salt, both of which are described as cosmetic biocides for use in cosmetic formulations. In an earlier Cosmetic Ingredient Review (CIR) safety assessment, the available data were found insufficient to support safety. Currently, some uses are reported to the Food and Drug Administration (FDA) by industry, but industry reports to CIR indicate no use. In Europe, Oxyquinoline and Oxyquinoline Sulfate are accepted for use as stabilizers for hydrogen peroxide in rinse-off and leave-on hair care preparations, with concentration limitations. Oxyquinoline is metabolized and excreted in the urine as glucuronides. Oxyquinoline and Oxyquinoline Sulfate exhibit little acute or subchronic toxicity in animal studies. A 100-mg dose of Oxyquinoline was only slightly irritating to the eye. Oxyquinoline and Oxyquinoline Sulfate were genotoxic in certain Salmonella typhimirium strains with metabolic activation and in a mouse lymphoma assay. There was some evidence of increased chromosome aberrations in an in vitro study, and an increase in sister-chromatid exchanges (but not chromosome aberrations) in rats treated with Oxyquinoline, but no genotoxicity was found in a Drosophilia sex-linked recessive lethal test, mouse bone marrow micronucleus test, a rat bone marrow and hepatocyte micronucleus test, and unscheduled DNA synthesis in rat hepatocytes. Oxyquinoline did bind to DNA in the presence of liver enzymes. Although the International Agency for Research on Cancer concluded that the existing evidence is inadequate to determine carcinogenicity in animals, Oxyquinoline was noncarcinogenic in several rodent feeding studies, and newly available studies using genetically altered mice, in one case carrying the human c-Ha-ras gene, demonstrated that Oxyquinoline was not carcinogenic. In clinical tests, Oxyquinoline is neither an irritant nor a sensitizer when tested at 1% in petrolatum. The available data demonstrate that Oxyquinoline and Oxyquinoline Sulfate are safe as stabilizers for hydrogen peroxide in rinse-off hair care cosmetic products in the present practices of use. For leave-on cosmetic products, however, the absence of impurities and ultraviolet (UV) absorption data resulted in a finding that the available data are insufficient to support safety. The data needed in order to complete the safety assessment of Oxyquinoline and Oxyquinoline Sulfate in leave-on cosmetic products are (1) UV absorption data -- if significant absorption occurs, then photoirritation/photosensitization data will be needed; and (2) data on impurities.