The many phytochrome-mediated photoresponses of a seedling (Sinapis alba L., white seeded mustard) can be divided into 3 categories: positive, negative, and complex photoresponses. "Positive" photoresponses are those which are characterized by an initiation or a promotion of biosynthetic or growth processes (Morn% 1966b). Phytochrome-mediated anthocyanin synthesis is the prototype of ~ "positive" photoresponse. It has been shown in previous papers (e. g. LA~GE and Mo~, 1965; Moor et al., 1965) that "positive" photoresponses can be specifically inhibited by actinomycin D ~nd puromycin. It has been concluded that in the case of "positive" photoresponses P~0 (the active phytochrome) exerts its function through differential gene activation. --In the present paper it h~s been demonstrated that phytochrome-mediated "positive" photoresponses of the mustard seedling like "opening of the hypocotylar hook" and "unfolding of the cotyledons" can be inhibited by relatively low doses of actinomyein D and puromycin in very much the same way as anthocyanin synthesis or cotyledon enlargement is inhibited. It has been concluded that in these cases too the action of PT~0 must be attributed to ~n activation of "potentially active genes" in the manner postulated on the basis of the data on anthocyanin synthesis.