Cells treated with low concentrations of glutaraldehyde for a 10-sec interval were unable to incorporate 3H-leucine into TCA precipitable protein, respond to H-2 allogeneic cells in mixed lymphocyte reactions (MLR) and cell-mediated lympholysis (CML) assays, or display capping of cell surface immunoglobulin (Ig) with a fluoresceinated anti-Ig reagent. Such cells could stimulate and specifically block H-2 allogeneic CML activity but could not stimulate an H-2 allogeneic MLR response. Trinitrobenzenesulfonic acid (TNBS) treated spleen cells were used to sensitize syngeneic splenocytes into displaying a cytotoxic effect against trinitrophenyl (TNP)-modified target cells. Treatment of the stimulator cells with glutaraldehyde immediately after modification with TNBS did not impair their immunogenic activity. Similar treatment of TNP-modified concanavalin A-stimulated lymphoblasts that were used as inhibitors in a CML cold target competition assay allowed such cells to retain their antigenicity. Cells treated with glutaraldehyde before TNP-modification, however, were not antigenic in the cold target competition assay. These data are compatible with TNBS acting on plasma membrane molecules directly to cause cells to be antigenic and immunogenic in the CML assay rather than affecting internal cellular components.