In five heterosexual male bovine twins and three freemartins, all not older than three months, XX/XY chimerism in the blood and gonads has been found. One heterosexual male bovine triplet showed at the age of 18 or 22 months XX/XY chimerism in the blood but not in the testes. In another male co-twin of a freemartin, XX/XY chimerism has been detected in the blood and the testes (at biopsy) at the age of two months. In the same animal, the blood chimerism was still present at the age of 18 months, whereas the chimerism of the testes had vanished at that time. Therefore, it can be concluded that, in the course of time, the XX cells of the testes have been eliminated. This elimination, which possibly occurs in embryonic life, partially explains why male co-twins of freemartins do not have malformations of the genital organs and are fertile. Possibly, the XY cells of the testes have an eliminating influence against neighbouring cells with an XX karyotype. The eliminating effect could be directed at the two active X chromosomes of these germ cells. The hypothesis would also account for the observation that blood lymphocytes with an XX karyotype, which have, according to the Lyon hypothesis, only one active X chromosome, are not eliminated with time.