Direct in situ labeling of human spermatozoa was performed using the PRINS method. This technique is based on annealing of specific oligonucleotide primers, and subsequent primer extension by a Taq DNA polymerase. The reaction was carried out on a programmable temperature cycler, and labeling was obtained in a 1-hr reaction. The method was successfully tested with specific primers for chromosomes 13, 16, and 21. This suggests that PRINS may be a fast and reliable technique for detecting aneuploidies.