Conditions for obtaining reproducible light-induced reduction of a b-type cytochrome in membrane fractions from coleoptiles of dark-grown Zea mays L. include a glucose-glucose oxidase system that lowers O(2) tension and generates H(2)O(2), substrate amounts of ethylenediaminetetraacetic acid which, in some manner, facilitates photoreduction by both added flavin and the endogenous photoreceptor and a sample temperature below 10 C. Cytochrome reduction could be obtained by photoexcitation of either a tightly bound endogenous receptor, which is probably a flavin, or added riboflavin, flavin mononucleotide, or flavin adenine dinucleotide. The latter flavin was the least effective. The endogenous photoreceptor appears to be rather firmly bound to the membranes, suggesting that this association may also exist in vivo. When any of the above four photoreceptors or methylene blue were used to sensitize the reaction, a cytochrome with a reduced alpha-band near 560 nanometers and a Soret difference peak near 429 nanometers was the electron acceptor. This cytochrome could be clearly distinguished spectrally from other cytochromes that predominated in the membrane preparations.