1. Sterility in certain Friesian bulls is associated with a specific morphological abnormality affecting the acrosomes of their spermatozoa.
2. This abnormality is visible in living spermatozoa and is also visible in fixed spermatozoa when precautions are taken to preserve the morphology of the cell.
3. If such precautions are not taken, and the spermatozoa are subsequently stained by the nigtosin-eosin method, or with Heidenhain's iron haematoxylin, or when such spermatozoa are examined with the phase-contrast microscope, the abnormality is inconspicuous.
4. These difficulties do not arise when either the Giemsa stain or indian ink is used.