A variety of N-acetylmuramyl-peptides (or -amino acids) were prepared by condensation of benzyl N-acetyl-4, 6-O-brnzylidene-alpha-muramide with various peptide (or amino acid) benzyl esters by the dicyclohexylcarbodiimide--N-hydroxysuccinimide or ethylchlorocarbonate--N-methylmorpholine method and removal of the protecting groups by hyderogenolysis. N-Acetylmuramyl-L-alanyl-D-isoglutamine was identified as the minimum structural entity essential for the immunoadjuvant activities characteristic of bacterial cell walls. Consequently N-acetylmuramyl-L-alanine was not adjuvant active. The tetrapeptide portion of adjuvant-active N-acetylmuramyl-L-alanyl-D-isoglutaminyl-L-lysyl-D-alanine proved to be inert, at least in induction of delayed-type hypersensitivity. The possible adjuvant activities of various analogues or diastereomers of the above N-acetylmuramyl-dipeptide and related compounds were studied. N-Acetylmuramyl-L-alanyl-D-glutamic acid exhibited weak, but definite adjuvancy, but none of the others, including N-acetylmuramyl-L-alanyl-L-isoglutamine, N-acetylmuramyl-L-alanyl-D-glutamine and N-acetylmuramyl-L-alanyl-D-isoasparagine, had any adjuvant activity. This clearly indicated the importance of the configuration of the glutamic acid residue or its amides, i.e. the presence of the D-isoglutamine residue in the N-acetylmuramyl-dipeptide, for manifestation of adjuvant activities in stimulation of both antibody-mediated and cell-mediated immune responses. Neither N-acetylmuramyl-D-isoglutamine nor N-acetylmuramyl-D-alanine had any adjuvancy.