A method to gently and permanently relax or straighten hair utilizes a protease enzyme, and preferably the enzyme kerA. The hair is optionally treated with a swelling agent, after which it is treated with the enzyme in an aqueous solution. The hair is then physically manipulated, such as by ironing, to remove unwanted curl, wave, and or kink. The enzyme has the ability to cleave inter-peptide bonds, allowing the hair fiber to be relaxed or straightened with less damage to the fiber than would have occurred using traditional or existing straightening methods.