Microperoxisomes were identified in the franulosa lutein cells from the corpora lutea of rhesus monkeys (Macaca mulatta). These organelles were histochemically visualized in aldehyde-fixed tissues icubated in alkaline 3,3'-diaminobenzidine (DAB). The DAB staining of the microperoxisomes was abolished when the tissues were preincubated in specific inhibitors for catalse or when the H2O2 was omitted from the DAB medium. Microperoxisomes were differentiated from primary lysosmes by the Gomori acid phosphatase staining. Tortuous undulating agranular endoplasmic reticulum (ER) was usually closely associated with microperoxisomes. Those regions of the granular ER which were closely associated with microperoxosomes lacked ribsomes. Micropersoxisomes were often contiguous with lipid droplets, and in some instances the limiting membrane of the moroperosisomes appeared discontinous at the point of contiguity, and the DAB staining substance diffused onto the surface of the lipid droplet. In these instances, the adjacent area of the lipid droplet showed electron-lucent staining.