Abstract: The present invention discloses a method and aqueous composition for hardening and strengthening the keratinized appendages of mammals, and the nail keratin of humans, in particular. The aqueous keratin hardening and strengthening composition includes an effective amount of fluoride ion. The topical application of the fluoride ion is effected from an aqueous cosmetic vehicle capable of maintaining moist contact with the keratin for a sufficient period of time in the method practiced. Auxiliary keratin-strengthening agents are also disclosed.