An in vitro fertilization assay employing zona-free hamster embryos was used to investigate human spermatozoal fertilitzing ability. Yanaghimarchi et al.(1976) first introduced this cross species fertilzation technique, with its application as a diagnostic tool for male infertility. Human spermatozoa were preincubated for 3 to 4 hrs in B W W medium at concentration of $4{ imes}10^6$ sperm/ml prior to the addition to zona-free hamster embryos. After 3 hrs, human sperm was evaluated for fertilizing potential by the presence of swelling or decondencing sperm head in the cytoplasm. The results of penetration rates for sperm were as follow : 1. The average penetration rate of a 7 fertile donor group was $47.8{pm}27.67%$(Range 14.3-98.0%) 2. The average penetration rate of 12 infertile patients with normal semen analysis was $21.7{pm}26.9%$(Range 0-38.8%) 3. The average penetration rate of 10 infertile patients with semen abnormalities was $6.1{pm}8.1%$(Range 0-25%)