SUMMARYMouse H-2 isoantigens have been solubilized from lymphoid cell-derived lipoprotein by autolysis. The soluble material was purified on P300 Bio-Gel and DEAE-Sephadex and activity then shown to be associated with the single component revealed on polyacrylamide gel electrophoresis. The product had about 500 times the specific activity of the starting material. Evidence is provided to show that several H-2 specificities are present on the same molecule and that in material prepared from F1hybrid mice at least some hybrid molecules exist, carrying specificities derived from both parental strains. The product did not react in any of several non-H-2 test systems.