To evaluate the usefulness of the determination of acrosome reaction (AR) at 2 and 4 hours of incubation for predicting sperm fertilizing capacity in vitro, semen samples obtained from 45 partners of couples participating in the IVF (in vitro fertilization) program were examined. The population was divided into 2 groups, depending on the fertilization rate in the IVF program: Group A, with a good fertilizing capacity (> or = 50% fertilized oocytes); and Group B, with a poor fertilizing capacity (< 50% fertilized oocytes). The rate of acrosome reaction (%AR) was examined using the fluorescein isothiocyanate-conjugated Concanavalin A lectin method. The efficacy of this procedure was compared with that of the zona-free hamster oocyte human sperm penetration assay (SPA) and sperm characteristics (SC) that appear in routine semen analysis. The sensitivities in the test of AR, SPA and SC were 91%, 86% and 82%, respectively. However, their specificities were 83%, 48% and 43%, respectively. Therefore, the screening procedure using the determination of acrosome reaction might be a useful tool in predicting IVF results.