OBJECTIVE To evaluate, using the hemizona assay (HZA), whether egg yolk treatment of human sperm enhances binding to the human zona pellucida in vitro and to determine whether such a treatment is as efficient as the standard swim-up procedure for promoting sperm binding ability. METHODS Ejaculates were divided into aliquots and half incubated at 37 degrees C for 21 hours in standard culture medium or combined with buffered medium containing chicken egg yolk and stored at 4 degrees C for 21 hours. A second set of ejaculates were split and processed by a standard 1-hour rise alone or treated with egg yolk medium. METHODS University teaching hospital. METHODS Two healthy sperm donors. METHODS The number of sperm tightly bound to the hemizona were measured and compared between the groups. RESULTS A significant increase (P less than 0.0001) in hemizona binding (n = 46) for egg yolk treatment (90.1 +/- 9.8; range 7 to 258) as compared with standard culture medium (53.0 +/- 8.8; range 0 to 228) was observed. Similarly, a significant increase (P less than 0.0001) in binding (n = 37) for egg yolk treatment (74.9 +/- 8.2; range 7 to 219) as compared with samples obtained with a sperm rise (37.1 +/- 5.7; range 2 to 122). CONCLUSIONS Treatment of human spermatozoa with an egg yolk medium at 4 degrees C overnight significantly increases sperm binding in the HZA.