An investigation has been made of the value of skin grafting for discriminating between monozygotic and dizygotic cattle twins, in accordance with the general rule that skin grafts exchanged between genetically dissimilar mammals are rapidly destroyed. Skin was accordingly interchanged between the cattle of the following degrees of affinity: - (1) unrelated animals, individually of twin birth; animals related only by being of the same pedigree breed, and individually of separate birth; (2) full siblings of separate birth; (3) dizygotic twins of unlike sex; (4) twins of similar sex classified by ordinary phenotypic appraisal as dizygotic; (5) twins classified by ordinary phenotypic appraisal as monozygotic. Both monozygotic and dizygotic twins are tolerant to grafts of each other's skin. Skin grafts interchanged between animals of the other degrees of affinity are very rapidly destroyed. Not all dizygotic twin pairs are completely tolerant to grafts of each other's skin, and examples of chronic low-grade immunity reactions are reported. It is suggested that the anomalous tolerance of dizygotie cattle twins to grafts of each other's skin has the same origin as their conformity of immunological blood types.