A microspectrophotometric study of the DNA content of carcinoma-in-situ germ cells in the testes of eight infertile men revealed an aneuploid DNA distribution pattern. The results support the assumption that carcinoma-in-situ germ cells are of a malignant nature. The findings also indicate that the DNA content can be used to discriminate between carcinoma-in-situ germ cells and spermatogonia.