BACKGROUND/AIMS Testing of sunscreen products requires application of uniform films of product of defined thickness to test volunteers. In spite of the seeming importance of product application to defining sunscreen efficacy, there have been few studies determining how well uniformity is achieved. The purpose of this work was to evaluate the uniformity of sunscreen products of different sun protection factors (SPFs) and vehicles on a variety of substrates by in vitro testing techniques. The results of a variety of testing strategies are reported. METHODS Five commercial sunscreen products of labelled SPF 4-50 were tested using a variety of substrates: Transpore Tape, Vitro-Skin trade mark, and lambskin condom. Two experienced sunscreen testers applied the products. In vitro SPFs were determined using an Optometric 290 analyser or an Optronic Laboratories OL754 spectroradiometer configured for this application. RESULTS SPFs for several locations on each film applied to a substrate were determined and the mean SPF and RSD percentage of the mean calculated. For all substrates and testing techniques the average RSD percentage was 18.6, with a range of 10-40%. CONCLUSION The expected uncertainty of SPF due to product application non-uniformity is 20% when applied under optimal test conditions. Clearly SPF tests reported by different laboratories must exhibit significant variability, because of product application non-uniformity.