A simple method is described for differentiating the acrosome in living mammalian spermatozoa and spermatids by induced fluorescence with acridine orange. Observations were made on spermatozoa from man, the bull, the rabbit, the hedgehog, the greater bush-baby, the greater horseshoe bat and eight species of rodent, and on spermatids from five species of rodent. With the exception of human spermatozoa, nuclei fluoresced bright apple green and acrosomes fluoresced bright red where they extend beyond the nucleus, and yellow or yellow-green where they overlap the nucleus. Human spermatozoa fluoresced entirely green, yellow-green or red and actively motile spermatozoa of each colour were seen simultaneously. The significance of the results and some problems associated with the method are discussed.
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