N-Acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and thirteen new analogs were synthesized by the conventional organic chemical procedure using dicyclohexylcarbodiimide-N-hydroxy-5-norbornene-2,3-dicarboximide as a coupling agent. Their ability to induce delayed-type hypersensitivity to N-acetyl-3-(4-arsonophenylazo)-L-tyrosine in guinea pigs was assayed. The results indicate that the presence of an α-amino