A study was made of the photoprotective effect of a sulfonated hydroxyphenylbenzotriazole light stabilizer in wool fabric. Investigations of the absorption, luminescence quenching, and photostabilizing properties of the benzotriazole showed that optical screening is the key mode of operation. Excited-state quenching is of little practical importance. The protective effect of the stabilizer is strongly dependent on the exposure conditions. See also 20-555. -- AATA