Lipofuscin accumulation in the testis of C57BL/6J mice ranging in age from 4 to 39 mo. was investigated by spectrophotofluorometric and electron microscopic techniques. Chloroform-methanol extracts of the whole organ contained fluorescent substances which increased linearly until about 24 mo. Thereafter, no further increases could be observed. Electron microscopy showed spermatogonia which were completely free of lipofuscin, in striking contrast with both Sertoli and interstitial cells of Leydig, which, in old animals, had accumulated very large amounts of pigment. In the Sertoli cells, a process of mitochondrial vacuolation and densification was apparently linked with the genesis of lipofuscin. On the other hand, in the interstitial cells, densification of lipid droplets played a role in the formation of most pigment granules.