This study describes some of the parameters that quantitatively or qualitatively influence the immunogenicity in guinea pigs of synthetic lipid antigens: phosphatidylethanolamine (PE) derivatives in which the amino (N) group has been substituted with either dinitrophenyl (DNP), dinitrophenylaminocaproyl (DNP-Cap), fluoresceinthiocarbamyl (Fl), or mono (p-azobenzenearsonic acid) tyrosyl (ABA-Tyr) residues. Previous experiments have shown that the non-covalent insertion of DNP-Cap-PE and ABA-Tyr-PE into the same lipid bilayers of sphingomyelin-cholesterol-dicetylphosphate liposomes markedly enhanced anti-DNP -Cap antibody formation over that produced by liposomes sensitized with only DNP-Cap-PE. The humoral response to Fl-PE and DNP-PE-sensitized liposomes is also augmented by the simultaneous incorporation of ABA-Tyr-PE. Moreover, micelles containing both DNP-Cap-PE and ABA-Tyr-PE induce more antibodies to the DNP-Cap determinant than do micelles of DNP-Cap-PE alone, or a mixture of DNP-Cap-PE and ABA-Tyr-PE micelles. Nevertheless, in regard to a humoral response, liposomes were more potent immunogens than were their micellar counterparts. Of all the N-substituted derivatives examined so far, ABA-Tyr-PE is unique in that it can elicit cell-mediated immunity in addition to antibodies. The cellular response to ABA-Tyr-PE is not, however, stimulated by incorporation into liposomal bilayers and requires administration of either micelles or liposomes in complete Freund's adjuvant. In contrast, the ability of ABA-Tyr-PE to enhance a humoral response to another N-substituted PE derivative present in the same immunogen is also observed when the latter are given with incomplete Freund's adjuvant. The relationship of these findings to the immunogenicity of naturally occurring lipid antigens, as well as conventional immunogens having at least one determinant covalently attached to a protein carrier, is discussed.