This study was designed to purify the mouse major histocompatibility complex antigen (H-2Ag). The detergent was used to extract the crude H-2Ag, and monoclonal antibodies affinity chromatography was applied to purify H-2 antigen. A 45 kd heavy chain and a 12 kd light chain from the purified proteins were shown by electrophoresis. Besides, the pure H-2 antigen was found to have immunological and biological activity. This method should be useful in purifying large quantities of H-2Ag for the researches in transplantation and functional analysis of H-2 antigen.