Ratios of sex-chromosome chimerism were determined, by cytogenetic analysis of peripheral leukocyte cultures, in 29 females and 19 males from 27 heterosexual bovine multiple births. A wide range existed in the degree of chimerism between multiple-birth sets, but, within sets, the percentage of XY cells in freemartins and their male co-sibs was positively correlated (r = 0.97, P < 0.001). The mean sex-chromosome ratio in both sexes approached unity. Since the chimeric relationship within sets was parallel rather than reciprocal, it is suggested that previous proposals do not adequately explain the method through which the chimeric variability arises. Reproductive tracts were recovered from 17 sacrificed freemartins, and there appeared to be no definite relationship between degree of virilization and percentage of leukocytes of male origin.