Spontaneous Leydig cell tumors (interstitial cell tumors) of the testis arising in aged rats have been studied by cytochemical procedures for identification of microbodies. Large numbers of microbodies with diameters ranging from 0.1 to 0.4 µ were present in tumor cells. The size of the tumor did not appear to have any effect on the microbody number in neoplastic Leydig cells. Examination of large numbers of microbodies in tumor cells revealed continuities between adjacent microbodies. The microbodies in Leydig cell tumors lacked a typical crystalline core or nucleoid, but some of the elongated forms contained cylindrical or tubular structures with periodicity. The nature of these inclusions remains obscure because urate oxidase activity was not detected in these tumors, by either biochemical or histochemical methods. In view of the presence of numerous microbodies, it is suggested that these tumors serve as a rich source of microbodies for studying their functional significance in Leydig cells of the testis.