A long-chain linear mono-benzylpenicilloyl (BPO) oligoamide and a succinoylated mono-BPO decalysine were tested in BALB/c mice for suppression of IgE and IgG1 antibody formation. Both compounds were available with either a free C-terminal end or were C-terminally linked to a hydrophobic 3 beta-cholestanyl residue. Only the sterol-containing derivatives suppressed hapten-specific IgE and IgG1 responses. Substantial suppression was obtained when the compounds were administered before primary or secondary, but not later immunizations. In an adoptive cell transfer experiment, spleen cells from tolerized animals actively suppressed anti-BPO IgE antibody formation of immune spleen cells. This effect was reversed by pretreatment of the tolerized spleen cells with anti-Lyt-2.2 antibody plus complement. The requirement for macrophages in the induction of T suppressor cells was demonstrated by injecting antigen-pulsed macrophages into naive recipients; upon immunization, only mice treated with tolerogen-pulsed macrophages showed suppressed anti-BPO IgE responses. It is suggested that lipid modification of antigens alters their processing and presentation by macrophages in a manner that leads to the induction of T suppressor cells. Injection of the cholestanyl derivatives into passively sensitized guinea pigs elicited anaphylactic reactions. By immune precipitation analysis and molecular weight estimation, these derivatives were shown to form micelles in aqueous solution. Therefore, the anaphylactic response appeared to be due to their behavior as multivalent antigens.