The expression of the mu-opioid receptor in human sperm cells was investigated by immunocytochemistry and immunoblot analyses. Results demonstrated that the receptor is localized on the acrosomal region and the neck portion of the sperm head. These results suggest a possible role for the mu-opioid receptor in mediating the action of endogenous opioid peptides on sperm cells during the complex and poorly characterized cellular events that enable spermatozoa to fertilize oocytes.