Oocytes with adhering follicle cells were sampled from ovaries obtained from 11 GPI-1A----GPI-1B chimaeras, comprising 10 females and 1 hermaphrodite. GPI analysis of individual oocytes revealed a marked bias towards the GPI-1B component in the germ line of this chimaeric combination. GPI-1B XY oocytes were identified in the ovary from the hermaphrodite, the bias towards the GPI-1B germ line perhaps helping to counterbalance the normally severe selection against XY oocytes. GPI analysis of follicle cells revealed a much more balanced contribution of the two components to this ovarian cell type. Importantly, GPI-1A follicle cells were identified in more than half the follicles from an XX----XY female in which the GPI-1A component was XY, supporting an earlier conclusion of Ford et al. (1974) that XY cells can contribute to the follicles of XX----XY female mice. It is suggested that XY cells can be recruited to form follicle cells in XX----XY chimaeras when there is a developmental mismatch between the two components, such that an ovary-determining signal produced by the XX component pre-empts the testis-determining action of the Y.