Assessing the ability of human spermatozoa to acquire fertilizing potential (capacitation) by stimulating exocytosis of the contents of the acrosome (acrosome reaction) is thought to have diagnostic potential (De Jonge, Reprod Med Rev 3:159-178, 1994). Calcium-mobilizing agents, such as calcium ionophores (A23187) and progesterone, stimulate the acrosome reaction in vitro (Brucker and Lipford, Hum Reprod Update 1:51-62, 1995). Acrosomal status is easily detected using Pisum sativum Agglutinin labeled with fluorescein isothiocyanate (Cross and Meizel, Biol Reprod 41:635-641, 1989). Herein we describe a procedure for assessing capacitation and the acrosome reaction of human spermatozoa in vitro.