The effect of maitotoxin (MTX), the most potent marine toxin produced by dinoflagellate, was investigated by monitoring the penetration ability of spermatozoa into oocytes. Bull spermatozoa were incubated in BO media with or without caffeine and/or calcium with final concentrations of 0.469, 0.938 and 1.875 ng/ml of MTX for 5 or 30 min. After MTX treatment, zona-free hamster oocytes were inseminated with the spermatozoa. When spermatozoa were treated with MTX in the medium without calcium, they failed to undergo the acrosome reaction and did not penetrate the oocytes. Caffeine stimulated the penetration ability of spermatozoa treated with MTX. When spermatozoa were treated with 1.875 ng/ml MTX, their motility was inhibited and they failed to penetrate into oocytes. Penetration of oocytes by spermatozoa treated with 0.938 ng/ml MTX for 5 min was enhanced. MTX treated spermatozoa began to penetrate oocytes within 15 min after insemination and achieved total penetration within 3 h after insemination. These results suggest that MTX may be used as a substance to induce sperm capacitation and to demonstrate a mechanism of acrosome reaction.