We have used DNA-DNA in-situ hybridization with a DNA probe for the human Y-chromosome to distinguish between male and female human pre-embryos. Both biotinylated and tritiated Y-probes worked well on control cell cultures where 100 interphase nuclei were scored. Morphologically normal pre-embryos could be sexed with confidence with the tritiated Y-probe but the biotin results were less reliable (although only a few pre-embryos were analysed in this way). Early cleavage stage pre-embryos had large nuclei with relatively diffuse Y-bodies and were more difficult to score with the biotinylated Y-probe. Morphologically abnormal pre-embryos often had large nuclei with multiple Y-bodies (presumably polyploid nuclei) or small nuclei with no Y-bodies (possibly fragmenting nuclei). In all, 38 cleaving and two non-cleaving pre-embryos were analysed. The incidence of false positive and false negative cells seen after hybridization of tritiated Y-probes to control lymphocyte cultures suggests that it should normally be possible to distinguish morphologically normal male and female pre-embryos with samples of three to six interphase nuclei.