Karyotyping and blood grouping methods were used to identify sheep twin chimaeras. Evidence that an exchange of blood cell precursors (the origin of chimaerism) had taken place in utero was obtained by examining lymphocytes in culture and finding the chromosomes of both sexes in one individual, or by finding admixture of red cell antigens, haemoglobin or 'X' protein. Where chimaerism of sex chromosomes was found the pairs had identical red cell types, but two separate populations of red cells were not always identifiable. The four females in the pairs studied were freemartins. No correlation was found between the relative proportions of the two red cell populations and those of the two white cell populations. In one pair of chimaeric ewes, breeding tests showed that the major red cell populations in each case were the true genetic type. In the freemartins no correlation was found between the degree of masculinity and the numbers of male lymphocytes. A possible correlation of masculinity with red cell proportions is discussed.