An antigen-specific T suppressor cell clone isolated from a CBA/J mouse tolerized to low doses of bovine serum albumin (BSA) has previously been analyzed with regard to its effector functions. The T suppressor cell clone HF1 specifically inhibits T helper cell responses to the antigen. It also has characteristic cytolytic activity which can neither be classified as cytotoxic T cell nor as natural killer cell activity. Since this lytic capacity might be of relevance in immunoregulation, it has now been studied in more detail. For that purpose BSA-specific T cell lines have been isolated from immune CBA/J mice in order to test them in 51Cr-release assays as possible targets for HF1 T suppressor cells. Two T cell lines, both BSA specific and restricted to recognition of I-Ek major histocompatibility complex determinants, have been selected for the studies because one is a helper cell (83/1), the other a suppressor cell type (83/2). HF1 T cells are able to lyse cells of line 83/1 but not those of line 83/2. Control experiments show that 83/1 cells are not a natural killer cell target and that on the other hand 83/2 cells are susceptible to lysis in an alloreactive BALB/c anti-CBA/J cytotoxic T cell response. The extent of lysis of 83/1 T cells by HF1 T cells changes with time after antigenic stimulation. The lysis is based on direct effector: target cell interaction and not caused by soluble mediators. The data are discussed with regard to the effector function of a type of T suppressor cells which expresses I-A and I-E molecules and whose proliferation is restricted to the recognition of I-A or I-E determinants.