The administration of cyclophosphamide to nonobese diabetic (NOD) male mice produces a rapid progression to overt diabetes (>70%) with severe insulitis in 2–3 wk, whereas none of the untreated control NOD male mice became diabetic. When the thin sections of islets from the NOD male mice, which received silica for the preservation of islets and subsequently cyclophosphamide, were examined under the electron microscope, clusters of endogenous retrovirus-like particles (type A) were frequently found in the β-cells. In contrast, retroviruslike particles were rarely found in the β-cells from NOD male mice that received only silica. Other endocrine cells, including α-, δ-, pancreatic polypeptideproducing, and exocrine acinar cells, did not contain such viruslike particles. These viruslike particles were also not found in spleen, liver, or kidney in either cyclophosphamide-treated or untreated NOD male mice. There was a clear correlation between the presence of retrovirus-like particles in the β-cells and insulitis lesions in the cyclophosphamide-treated mice. On the basis of these observations, we conclude that the β-cell–specific expression of endogenous retrovirus (like particles) is associated with the development of insulitis and diabetes in NOD mice.