The specificity of C57BL/10 cytotoxic effector cells generated by in vitro sensitization with autologous spleen cells modified with a series of related nitrophenyl compounds was investigated. The failure of trinitrophenyl (TNP)-sensitized effector cells to lyse TNP-beta-alanylglycylglycyl(AGG)-modified target cells is presented as evidence contradicting the intimacy or dual receptor model or T-cell recognition in its simplest form. Data are also shown indicating that sensitization with N-(3-nitro-4-hydroxy-5-iodophenylacetyl)-AGG-modified stimulating cells generates noncross-reacting clones of cytotoxic effector cells.