Cells from mice inoculated with picryl sulphonic acid (PSA cells), which contain suppressor T cells for contact sensitivity to picryl chloride, were examined for their ability to alter antibody responses of normal mice. These cells did not influence antibody or plaque-forming cell (PFC) production accompanying contact sensitivity reactions produced by painting with picryl chloride but reduced IgG antibody and indirect PFC responses to conjugates of trinitrophenyl (TNP) bovine serum albumin and ovalbumin. IgG responses to TNP or new antigenic determinants of TNP-mouse serum albumin were not affected by PSA cells. The PSA cells required several weeks to produce reductions of responses and only reduced responses to optimal doses of antigen. When the injection of antigen was delayed until several weeks after the injection of PSA cells rapid reductions of responses were found but these were short-lived. The inhibition was specific for TNP proteins although responses to hapten and carrier were reduced. Evidence is presented to show that the inhibition was mediated by an adherent macrophage-like cell rather than a T cell. The inhibitory activity was resistant to irradiation and anti-theta treatment but was removed by glutaraldehyde treatment and cotton wool filtration.