Previous studies have shown that suppression of 2,4-dinitrofluorobenzene (DNFB) contact sensitivity by soluble suppressor factor (SSF) requires that the donor of immune lymph node (LN) cells and of SSF share either the H-2K and/or H-2D region of the major histocompatibility complex. Thus, target or acceptor molecules for SSF appear to be coded for by genes within the H-2K and H-2D loci. Experiments were done to investigate the nature of these target molecules and to determine what cell types expressed them. It was found that purified lymph node T cells are suppressed by SSF indicating that T cells express the acceptor molecules. Adsorption experiments showed that the only cells capable of adsorbing the suppressor factor are DNFB-immune T cells from donors which share with the factor-producing strain either the H-2K or H-2D locus. This adsorption can be specifically blocked by pretreating the immune LN cells with antibodies directed against H-2K and/or H-2D determinants or against the hapten DNP but not by antibodies against Ia or theta-antigens. Collectively, these results indicate that the target molecules are expressed only by DNFB-immune T cells and are comprised of histocompatibility antigens associated with DNP.