In a previous study we demonstrated that antiviral response against the coliphage MS-2 can be elicited by immunization with a synthetic antigen consisting of a conjugate (P2-A -- L) of a synthetic fragment (P2) of the virus coat protein attached to a synthetic polymeric carrier. The antiviral response was induced when the antigen was administered in complete Freund's adjuvant or when it was administered in incomplete adjuvant, provided that a peptidoglycan was covalently attached to it. In the present study we demonstrate the adjuvant effect of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) in this system. Immunization with a mixture of MDP and P2-A -- L brought about only slight enhancement in the titer of neutralizing antibodies, as compared to the immunization with P2-A -- L in saline. The best results were achieved when the MDP was chemically conjugated to P2-A -- L. This completely synthetic material, when administered in aqueous solution, yielded highly inactivating antiserum with a titer similar to that obtained with complete Freund's adjuvant in the absence of MDP. MDP-P2-A -- L elicited also a humoral immune response to MDP, but with much lower titer than that induced by complete Freund's adjuvant containing P2-A -- L only. It was also observed that the capacity of MDP-P2-A -- L to increase resistance against infection was more than a 100-fold greater than that of unconjugated MDP.