Detergent-solubilized, affinity-purified class I and II MHC antigens can be immobilized on plastic by a simple detergent dilution procedure. The bound antigens retain allogeneic serological determinants and can be precisely quantitated by ELISA. The ability to quantitate immobilized antigen greatly facilitates purification by affinity chromatography. It is shown that differential elution can be used to highly enrich H-2Kd and Dd antigens from a single monoclonal antibody column which binds both. Binding of membrane proteins (class I, class II and plasma membrane protein) to plastic could be distinguished from binding of non-membrane proteins (bovine serum albumin and immunoglobulin) in competition studies and by comparison of their susceptibilities to inhibition by detergent. These contrasting properties suggested that MHC proteins may bind via their exposed hydrophobic regions and thus be oriented on the plastic surface. This was supported by the demonstration that immobilized class I is effectively recognized by alloantigen-specific cloned CTL to trigger the antigen-dependent degranulation response. Direct immobilization of MHC antigens, and probably other membrane proteins, provides an effective approach to the study of T cell recognition and triggering by physiological ligands.