Using a panel of congenic resistant mice differng from C57BL/10ScSn at theH-1,H-3,H-4,H-7,H-8,H-9,H-10,H-11,H-12, andH-13 histocompatibility loci, the median survival times of skin allografts from and to C57BL/10ScSn were obtained. The following observations were made: 1. The strengths of the barriers imposed by the non-H-2 histocompatibility loci were quite variable, the median survival times for the various loci ranging from 15 to > 300 days. 2. The reciprocal graft rejections across non-H-2 burriers were often quite differet. BIOBY mice rejected C57BL/10ScSn skin with a median survival time of 15 days. C57BL/10ScSn mice rejected B10BY skin with a median survival time > 250 days. 3. The longer the median survival time, the greater was the range in survival times of individual grafts. 4. The rejection time of females of a given strain was frequently shorter than that of males. 5. Preimmunization with donor thymocytes increased survival of recipients of tumor allografts and, except in the case of the weakest histocompatibility differences, shortened the survival of skin allografts. 6. Injection at intervals of 1 week of 1, 2, and 4 X 105thymocytes was less effective in protecting against a subsequent tumor allograft than 10 times these numbers of cells. However, an additional increase the cell dosage did not further increase the protection. 7. Skin grafted 7 days after immunization was rejected faster than skin grafted 14 days after immunization. Skin grafted after triple immunization was rejected faster than skin grafted after single immunization. 8. The strengths of rejection of skin and tumor across non-H-2 histocompatibility barriers paralleled one another.