In order to compare fluorescent peanut (Arachis hypogaea) agglutinin lectin and GB24 antibody (specific for the inner acrosomal membrane) techniques for the assessment of acrosome reaction, both methods were applied on semen specimens obtained from patients undergoing in-vitro fertilization (IVF). The acrosome status was evaluated after a 4 h incubation in B2 medium with and without calcium ionophore A23187. Results obtained with both techniques were compared and studied as a function of IVF outcome. The percentage of spontaneous acrosome-reacted spermatozoa was higher when assessed by lectin than by GB24 (19 +/- 2% versus 11 +/- 1%; P < 0.001). The difference between the two methods (lectins minus GB24) was significantly higher in abnormal than in normal spermatozoa (10 +/- 2% versus 4 +/- 2%; P < 0.05), but did not significantly correlate with the percentage of acrosomes with abnormal morphology (r = 0.28; NS). When studied in relation to the IVF results, the response to A23187 was higher in successes than in failures (45 +/- 2% versus 34 +/- 4%; P < 0.05) but there was no significant difference between methods. Thus the assessment of acrosome reaction is strongly influenced by the method used, particularly in abnormal spermatozoa. Since the results obtained with lectins were higher in abnormal spermatozoa, GB24 seems to be more effective for assessment of true acrosome reaction.