Concentrating human sperm before cryopreservation.

To improve the quality of cryo-preserved sperm, ejaculated semen was concentrated prior to freezing by means of continuous-step density gradient centrifugation. Freezing was simplified by employing liquid nitrogen vapor with KS-II cryo-medium. The original semen (n = 32, 41 +/- 13 x 10(6)/ml, 17 +/- 11% motility) was improved by the processing to be 68 +/- 29 x 10(6)/ml, 55 +/- 19% motility. Even after the specimens were thawed, sperm concentrations were similar to those of the original semen and 39 +/- 18% motility remained with a mean survival rate of 74 +/- 7.7%. The improved sperm motility and higher survival rate resulted in better post-thaw sperm quality than those of the original semen.