1. In etiolated wheat (Triticum aestivum L.) leaves, the development of the microbody enzymes catalase, hydroxypyruvate reductase, and glycolate oxidase was specifically stimulated by short treatments of the seedlings with red light, although the increases were less than observed after treatment with continuous white light. A comparison of the effects of short red and far-red exposures indicated the involvement of phytochrome. 2. Continuous far-red light treatments also enhanced the development of microbody enzymes. Catalase activity continued to increase at a high rate even after return from a prolonged far-red illumination to darkness, while the increase in the activities of glycolate oxidase and hydroxypyruvate reductase fell to the dark rates when the tissue was removed from the light. However, even at higher intensities of continuous far-red light the microbody enzymes reached only considerably lower activities than in white light. During continuous irradiation of equal quantum flux, the microbody enzymes reached higher activities in red than in far-red light, but the highest activities were observed in blue light, which had similar effects as white light. The quantitative difference between the effects of prolonged red or blue light depended also on the seed material and growing conditions. In the presence of the herbicide 3-amino-1,2,4-triazole the increase of glycolate-oxidase activity was reduced in red light but was affected much less, if at all, in blue light. 3. Continuous irradiations with all three light qualities used (red, far-red, blue) influenced the properties of the microbody particles to form a distinct band sharply confined close to an equilibrium density of 1.25 g cm(-3) on sucrose gradients which was not observed in preparations from plant material raised in complete darkness. In preparations from all light-grown plants a special peak in the activity profile of malate dehydrogenase was found in the microbody fraction while it was lacking on gradients from dark-grown leaves. The heights of the activities of malate dehydrogenase as well as of the other enzymes found in the microbody fractions from plants grown in either far-red, red, or blue light differed in the same way as did the activities from total leaf homogenates. 4. Glycolate oxidation by segments of intact leaf tissue was higher with tissue from light- than from dark-grown plants, but after light treatments of different spectral quality its magnitude did not correspond to the extractable activities of glycolate oxidase.