AbstractThe fine structure of the interstitial cells of Leydig has been examined in the squirrel monkey,Saimiri sciureus, during the non breeding portion of the year when the testes are essentially azoospermic. Interstitial cells are present in abundance as a layer, several cells thick, on the inner aspect of the tunica albuginea. Cytologically, the cells possess the characteristics usually associated with cells which synthesize and secrete steroid hormones: an abundant agranular endoplasmic reticulum in the form of anastomotic tubules; mitochon‐chondria with both lamellar and tubular cristae; only a few strands of granular endoplasmic reticulum; a Golgi apparatus and associated vesicles, some of which are of the coated variety; a varying population of lipid droplets and dense bodies. Some of the dense bodies appeared to be lipofuscin pigment. A previously undescribed inclusion was frequently encountered which consisted of arrays of a honeycomb like structure. Most frequently these structures were present near the cell periphery or in blunt protrusions of the cytoplas