RECENT observations1–4 of specific localized fluorescence in Y chromosomes stained with quinacrine mustard and quinacrine hydrochloride have provided a new parameter for the study of abnormal Y chromosomes. We have re-examined regularly prepared chromosome and buccal smear slides of at least 100 cells from two patients with a small Y chromosome5 and from patients with normal Y chromosomes. We used a modification of techniques described earlier1–4. We stained for 5 min in 0.5% quinacrine hydrochloride (Winthrop); washed in distilled water; differentiated for 3 min in citric acid-phosphate buffer, pH 5.5; washed and mounted slides in phosphate buffer, pH 7.4, and sealed with nail polish. Preparations were examined with a Leitz Ortholux microscope with a mercury arc lamp, standard fluorescence filters, and dark-field illumination, and were photographed with Ektachrome high speed film. Fluorescence persisted for several days. Full details of the technique will be published later.