The acrosome is an organelle that develops over the anterior half of the head in the spermatozoa (sperm cells) of many animals. It is a cap-like structure derived from the Golgi apparatus. Acrosome formation is fully completed 5-10 years after testicular maturation. In Eutherian mammals the acrosome contains digestive enzymes (including hyaluronidase andacrosin). These enzymes break down the outer membrane of the ovum, called the zona pellucida, allowing the haploidnucleus in the sperm cell to join with the haploid nucleus in the ovum.

This shedding of the acrosome, or acrosome reaction, can be stimulated in vitro by substances a sperm cell may encounter naturally such as progesterone or follicular fluid, as well as the more commonly used calcium ionophore A23187. This can be done to serve as a positive control when assessing the acrosome reaction of a sperm sample by flow cytometry[ orfluorescence microscopy.