SUMMARY Three methods were tested for fractionating H-2 antigens from a participate fraction of Sarcoma I using the 51Cr cytotoxic assay to measure antigenic activities. A principal difference among the fractionation procedures was the type of detergent employed to solubilize the antigens. A procedure using Triton X 100 was the least satisfactory. Those using Triton X 114 or cholate were equally effective, but the procedure involving the use of cholate was more convenient. Fractionation of antigens in detergent solutions with (NH4)2SO4 was not an efficient procedure. Digestion with trypsin further increased the specific activity of fractions obtained by treatment with detergents. The highest purification attained was a 10-fold increase in activity over that of the starting material. The yield of activity was usually about 60%. No evidence for separation of H-2 antigenic specificities was obtained. Detergent-solubilized preparations were retained wholly or in part on Sephadex and Agarose columns, but these procedures did not result in increased purity