Skin grafts were exchanged between partners of 35 chimeric twin pairs of cattle. Autografts were maintained indefinitely and third-party allografts were rejected within 14 days. However, only 30% of the twins retained their cotwin's allografts longer than 2 years; some grafts were viable after 5 years. The results indicate that chimeric twins exhibit a high degree of acquired immunological tolerance for the histocompatibility antigens of skin. Most of the second-set and all of the subsequent cotwin grafts were rejected. Furthermore, the differences in response between members of a twin pair noted for first-set grafts were not apparent with later grafts, suggesting that a chimera can mount a sufficient immune response to reject its cotwin's skin. Since these twins exhibited erythrocyte chimerism, it is concluded that they were completely and permanently tolerant to erythropoietic antigens but only partially tolerant to histocompatibility antigens of skin. Thus, communal circulation of twin fetuses which produces chimerism leads to different results for these different tissues. Apparently, cattle erythrocytes do not carry major histocompatibility antigens; therefore, chimerism of erythropoietic elements is independent of graft response. Some suggestion that lymphocytes of cattle do not carry all of the antigens carried by skin was obtained. The fate of cotwin grafts was not affected by whole body γ radiation, by chemicals that affect hematopoiesis, or by kidney grafts.