Abstract
The present article describes an occurrence of testicular microlithiasis in a cynomolgus monkey from a routine regulatory toxicology study. The monkey was from a negative control group. Microscopically, the lesion was characterized by multiple extracellular mineralized calculi within seminiferous tubular epithelia of both testes without any tissue reaction or abnormal condition such as cryptorchidism, testicular neoplasm, or hypogonadism. The present case is remarkable in that there is a paucity of reports on spontaneous testicular microlithiasis in nonhuman primates. It is hoped that this case report will help to facilitate the differentiation of spontaneous changes from induced changes in nonhuman primate toxicology studies that are designed to use limited numbers of animals.