Economics dictate that livestock producers will be under increasing pressure to optimise output. A technique for sex pre-selection could help by reducing the number of females required to produce a given number of progeny of the desired sex; the technique would be particularly useful to the dairy industry. Live mammalian sperm, stained with a vital dye and analysed by flow cytometry, show a bimodal fluorescence distribution. Such bimodality may represent two overlapping subpopulations of X- and Y-chromosome bearing sperm. To test this hypothesis, sperm from the two subpopulations were separated using the sorting capacity of a flow cytometer and were used for the insemination of suitably prepared females. The sex of the resulting progeny was determined either by anatomical criteria or by identification of the sex chromosomes by karyotyping. Insufficient data are available so far to provide statistically significant evidence in support of the hypothesis, but a preliminary sequential analysis indicates a progressive tendency towards significance.

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The proband is a three year old Maori girl born to a 17 year old mother and a 20 year old father. Pregnancy was uneventful with forceps delivery at term. Birth weight was 3340 g. No external abnormality was observed other than one extra digit on each foot which was ligated before discharge. At the age of three years her height is 98-0 cm and weight 16 kg. She has been assessed as mildly mentally re-