Human sperm chromosomes can be analyzed using the zona-free hamster egg system. In order to familiarize ourselves with this new technique and to investigate the chromosome aberration frequency in fertile and possibly infertile men, we performed sperm chromosome analysis in 39 successful experiments. The subjects were divided into two groups. Group 1 was healthy men with proven fertility. Group 2 was husbands of couples with unexplained infertility. Semen samples were prepared freshly or preserved in TEST-yolk buffer for 12 to 72 hours. In group 1 using fresh samples, the mean penetration rate was 67.40 +/- 14.08% (mean +/- 2SD), the maturation index was 1.33 +/- 0.31. For samples treated with yolk buffer, the mean penetration rate was 91.34 +/- 7.42%, and the maturation index was 1.58 +/- 0.42. There was a statistically significant increase in penetration after yolk buffer treatment (p less than 0.001). In group 1 there were 89 haploid chromosomes, four were aneuploidy (4.5%), and three with structural aberrations (3.4%). In group 2, for fresh samples, the mean penetration rate was 71.34 +/- 27.96%, and the maturation index was 1.37 +/- 0.25. For samples treated with yolk buffer, the penetration rate was 86.33 +/- 25.18%, and maturation index 1.40 +/- 0.23. In group 2, there were 135 haploid chromosomes, 6 were aneuploidy (4.4%), and 4 with structural aberrations (3.0%). There was no statistically significant difference between group 1 and group 2 in the penetration rate, maturation index and frequency of chromosome abnormalities.