The male factor in fertilization of rat eggs in vitro

Both epididymal and ejaculated spermatozoa give usable rates of fertilization of superovulated oocytes in vitro. The epididymal sperm gave higher rates, but ejaculated sperm were preferred because intra‐donor variation was lower than inter‐donor variation. A selected population of 14 male donors had normally distributed fertilizing capacity with a mean and standard deviation of 77 ± 14% (2,823 eggs). Successive ejaculates usually gave similar rates of fertilization. An attempt was made to eliminate dead and agglutinated cells from the ejaculated sperm by filtration through a column of glass beads. Despite the expected marked improvement in the average motility, the eluted spermatozoa were incapable of fertilizing. We concluded that the preferred source of spermatozoa for fertilization in vitro is ejaculated sperm. In the rat, motility and cell morphology are not adequate criteria for fertilizing capacity of treated spermatozoa.