An in vitro system for the induction and assay of hapten-specific thymus-derived cytotoxic cells has been developed. Mouse spleen cells can be induced to kill trinitrophenyl (TNP) coupled tumor cells. Immunoadsorbent columns specific for 1gM, 1gA, 1gGl, 1gG2a, 1gG2b, and 1gG3 immunoglobulins of mice have been prepared. These columns will be used to fractionate anti-TNP into subclasses, which will then be tested for their ability to block the cytotoxicity to TNP-coupled tumor cell targets. Free antibody and antibody complexed with various hapten- carrier conjugates will be examined for their blocking ability.