A nonradioactive in situ hybridization technique was applied to human gametes and abnormally fertilized or developed zygotes. Using haptenized chromosome-specific probes, visualization was obtained using immunocytochemistry to achieve a fluorescent stain on specific hybrids. Using a chromosome 1-specific DNA probe, almost all spermatozoa gave a positive result, i.e., one hybridization signal per cell could be observed. Furthermore, it was possible to identify sperm cells with two spots, sug gesting nondisjunction. Two cleavage arrested embryos from different patients showed both: two brightly fluorescent spots and two weaker spots with the same DNA probe. Using a Y-specific DNA probe the percentages of positive spermatozoa from the normal males ranged between 48.1% and 49.1%. In an embryo with four grossly haploid chromosome sets, three fluorescent spots were obtained with the Y-specific DNA probe, indicating the penetration of three spermatozoa.