The presence of Y bodies and similar fluorescent chromocentres in the interphase cells of 73 benign and malignant neoplastic lesions of males, and 69 of females, has been assessed in preparations stained with quinacrine dihydrochloride. In the male, Y bodies were usually present, although none was seen in 16 of the 65 malignant tumours. Sometimes the Y body was present in duplicate, for example, in some regions of a benign polyp of the colon and generally in 10 of the malignant tumours. The series includes 5 seminomata and 12 malignant testicular teratomata, all of which were positive for Y bodies.The tumours of females failed to show similar bodies, with 5 exceptions: one of 13 carcinomata of the ovary showed a body resembling the Y body in about half the cells (however, a similar body was seen in lymphocytes from this case) while a further carcinoma of probable ovarian origin, and 3 of 13 ovarian dermoids, showed a similar body though in less than 20% of the cells.Although quinacrine fluorescence studies on interphase tumour cells may be of value in suggesting the presence or absence of Y chromosomes, it is desirable that these studies be supplemented by the investigation of the fluorescence pattern of the metaphase chromosomes.