We have studied human spermatozoa from 24 normal, healthy unexposed men, 18 of whom were semen donors at the Sperm Bank in Turku, using multicolor fluorescence in situ hybridization with two chromosome-specific probes. The possible age-related increase in aneuploidy frequencies was assessed. Ten thousand spermatozoa were scored per individual for the presence of hyperploid, i.e., disomic and diploid, cells. The overall hybridization efficiency was 98.8%. The frequency of spermatozoa with two chromosome 1 signals was 11.+/-5.2/10,000. The frequency of spermatozoa with two chromosome 7 signals was 6.4+/-3.9/10,000. Diploidy was present in 15.0+/-8.9/10,000 spermatozoa. Interindividual variation was quite large. No statistically significant correlation between age of the donors (range=20-46 years) and the frequency of hyperploid spermatozoa was observed. The results give background information on the incidence of hyperploid spermatozoa in unexposed men and encourage the use of this novel technique for future studies on genetic effects in men exposed to potentially aneuploidogenic agents.