An invisible change mature sperm (spermatozoa) undergo to acquire accelerated movement and the ability to undergo the acrosome reaction. Brought about naturally when sperm swim up through the uterus and fallopian tubes, or brought about in the laboratory by spinning and washing the sperm through a series of solutions (in preparation for assisted conception).

If the acrosome is a balloon-like crash-helmet for the mature spermatozoon, the acrosome reaction is the bursting of that balloon, releasing enzymes that digest a path for the highly motile sperm (after capacitation) to push through the zona pellucida into the perivitelline space, where it can directly fertilise the egg.