Summary Blood and transferrin types of 96 pairs of cattle twins were investigated. For diagnosing zygosity, both transferrin and blood types were more efficient than either alone. In intrapair comparisons of 44 twin pairs with erythrocyte mosaicism, 18 pairs had different transferrin types and 22 pairs had identical types. Four pairs had similar but not identical types; they exhibited definite intensity differences in one or more iron-binding bands suggesting the possibility of transferrin chimerism.