Bloodtyping was performed for 708 pairs of twins to establish zygosity. All pairs discordant on one or more factors were classified as dizygotic, and the results obtained from this DZ sample were tested against the expected distribution of concordant and discordant DZ pairs for each major blood group. There was close correspondence between expected and obtained values for all groups except Rh. Equations were developed for determining the probability that a concordant twin pair would be DZ rather than MZ. The method is illustrated for cases where the parents have also been bloodtyped, and for the more typical case where parental data are not available. In the latter case, there is about a 3% chance that a pair of twins who are concordant on all the major blood groups will be DZ.