A marked variation in the concentration of sodium lauryl sulphate (SLS) used for irritant patch testing is found in the literature, but is hitherto unexplained. In the present study, 2 different qualities of SLS were tested clinically on healthy volunteers. The skin responses were evaluated by visual scoring as well as by non-invasive measurement of transepidermal water loss (TEWL), blood flow and oedema. A significant difference in the skin response to the 2 qualities was found both clinically and by non-invasive methods used for quantitation. 5 different qualities of SLS were investigated by high-performance liquid chromatography (HPLC). Marked discrepancies in the quantity of C12 carbon chains in the products were found, offering an explanation for the proven difference in the clinical response. It is concluded that only SLS qualities of high purity should be used for irritant patch testing, and that both the quality and the purity of SLS should be stated.