EJACULATED spermatozoa cannot be preserved satisfactorily by conventional fixation procedures for electron microscopy. Osmium tetroxide (OsO₄) fixation of crude ejaculate consistently produces a variety of artefacts such as separation of the plasma membrane from the acrosome, widening of nuclear vacuoles, erosion of the acrosome, and swelling of mitochondria^1–3. These alterations could be the consequence of the rapid destruction of the fixative by the proteins of the seminal plasma. Similar results are obtained whenever the semen is washed either in salt solutions^3,4 or in other media^5–7 before osmium tetroxide fixation. In these cases, the structural alterations of the sperm are probably caused by osmotic imbalances. Fixation with glutaraldehyde is also unsuccessful in that it is associated with the typical artefacts of this fixative (our unpublished work).