Laboratory of Reproductive Pharmacology, Department of Pharmacology, New York Medical College, Valhalla, New York 10595, U.S.A. (Received 3rd June 1974) The dye, ethidium bromide, markedly increases its fluorescence quantum efficiency when bound to double-stranded undenatured nucleic acid (Le Pecq, Yat & Paoletti, 1964; Le Pecq & Paoletti, 1966). When added to sperm suspensions, ethidium bromide binds almost exclusively to the sperm head and induces an intense fluorescence (Edelman & Millette, 1971). In this report, a method for DNA assay in human semen and washed sperm suspensions is described. The method takes advantage of these earlier findings and permits quantitative estimates of DNA and, indirectly, of sperm counts to be made in less than 15 min. Semen specimens were obtained from medical students. Sperm counts were made in quadruplicate using a haemocytometer, as described by Freund & Carol (1964). The procedure for preparing washed sperm suspensions and the composition of the
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