An in vitro technique for the prediction of percutaneous penetration/dermal absorption is recommended by the European SCCNFP (The Scientific Committee on Cosmetic Products and Non-Food Products intended for consumers) for the safety evaluation of particular ingredients. In 2002, this in vitro method became officially accepted at the OECD level and will be published as OECD Guideline 428. Examples are given for its routine application, demonstrating the bioavailability of cationic azo hair dyes out of an aqueous solution compared to data from a realistic standard hair dye formulation. Data from two direct hair dyes, BASIC BROWN 17 and BASIC YELLOW 57, demonstrate the results of applying this new in vitro technique without radiolabelled material. Direct hair dyes are frequently used in semi-permanent cosmetic hair colourations without the additional need for hydrogen peroxide. In the light of the in-house experience over about 5 years in using excised pig skin for measurements of the percutaneous penetration and dermal absorption of hair dyes, the technique was found to be successful and appropriate for reducing the number of test animals normally used for toxicological assessments.