Intermediary forms of microbodies, glyoxyperoxisomes, are most likely synthesized at the stage of replacement of glyoxysomes by peroxisomes during the greening of fatstoring cells of cucumber cotyledons. This hypothesis is supported by three lines of results: 1. The levels of both glycollate oxidase and malate synthase are increased in the microbody fractions upon illumination of cotyledons. 2. Both peroxisomal and glyoxysomal marker enzymes are localized in organelles which exhibit the same buoyant densities. 3. De novo synthesis of malate synthase takes place even when irradiation already leads to a drastic increase of glycollate oxidase. The amount of malate synthase synthesized is almost identical in the light-induced system and in the dark control.