Adequate acrosome reaction is one of the essential events that has to occur in successful mammalian fertilization. The purpose of the present study was to assess the acrosome reaction in human spermatozoa by means of iodine-labelled lectins concanavalin A and peanut agglutinin ([125I]Con A and [125I]PNA). Six sperm samples in the control (fertile) group were compared with 24 samples obtained from infertile patients. The acrosome reaction in both groups was induced in vitro by adding follicular fluid. Iodine-labelled lectins were bound to the sperm surface, and alteration in the binding capacity for [125I]PNA and [125I]Con A after induced acrosomal reaction was the main parameter for the prediction of acrosome reaction and fertilizing ability. It is hoped that with the availability of this test, in-vivo data may be accumulated.