A standardized ultraviolet erythema test in guinea-pigs is described in detail. The erythema reaction to 120 sec of UV irradiation was assessed after 1, 2 and 4 h. A direct quantitative comparison of the anti-erythema effect was made between phenylbutazone, indometacin, tolmetin, ketoprofen and alpha-methyl-4-(2-thienylcarbonyl)benzeneacetic acid (suprofen) in a specially designed experiment. All compounds were given by oral gavage 1 h before irradiation. Among the compounds studied suprofen was the most potent antagonist of ultraviolet-induced erythema in guinea-pigs, i.e. about 45 times tolmetin, 25 times phenylbutazone, 10 times indometacin and 5 times ketoprofen activity.