- Reciprocal skin grafts between the C57BL/6 (H-2b) mice and the mutant strain B6.C-H(Z1) (H-2ba), which differ only in a point mutation in the K region of the major histocompatibility complex, elicit rejection reactions similar to those elicited by skin allografts incompatible at multiple loci. Thyroid grafts in the same combinations are not rejected. Analyzing the immunogenicity of H(Z1) skin and thyroid tissues, we found that secondary H(Z1) thyroid grafts were rejected by C57BL/6 mice which had rejected H(Z1) skin tissue. On the other hand, pregrafting of C57BL/6 mice with H(Z1) thyroid did not induce a rejection reaction against a second-set H(Z1) thyroid transplant. These results indicate that gene products of the K region are expressed on thyroid cells and are recognized by memory lymphocytes induced by the H(Z1) skin. Yet, unlike the skin cells, the thyroid expressed H-2k-associated determinants in a nonimmunogenic form which can induce neither an actual rejection nor a state of memory.