XX/XY chimerism has been studied in the liver of 22 presumptive freemartin fetuses and in 12 male cotwins of multiple pregnancies, at an age of 39 to 62 days post-insemination. At these stages, the majority of the liver cells are hematopoietic cells. In these fetuses, the degree of chimerism varies from 0% to 55 %. Except for one case, the cells of the host predominate, i.e., the percentage of XY cells in freemartins is less than 50 % and is of the same order of magnitude as the number of XX cells in the male twin. According to several authors, freemartins and their male twins, when studied postnatally, possess the same percentage of XY (or XX) blood cells (variations from 1 % to almost 100%). These data suggest that the degree of chimerism of blood cells might change during development. If this is so, it seems unwarranted to compare chimerism in postnatal freemartins with the sex anomalies which appeared at an early fetal stage. The first signs of freemartinism (inhibition of gonadal growth and regression of the upper part of the Müllerian ducts) appear after day 48. In 15 freemartin fetuses of 49 to 62 days of age, the degree of inhibition of the left ovary and of the upper Müllerian ducts was compared with the percentage of XY cells present in the liver. The degree of inhibition of the ovarian volume and of the diameter of the upper Müllerian ducts was evaluated by reference to regression lines which had been previously calculated for a larger number of freemartins of the same age. There is no correlation between the percentage of XY cells and the degree of inhibition of the ovaries or of the Müllerian du