Twenty-two kinds of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) analogs were synthesized and their adjuvant activity on the induction of delayed-type hypersensitivity to ABA-N-acetyl-L-tyrosine was examined. The L-alanine residue of MDP could be replaced with certain other amino acid residues without loss of activity. The structureactivity relationship of these compounds is discussed. With respect to replacement of the L-alanine residue of MDP in connection with adjuvant activity, it was shown that (1) amino acids having a suitable side chain were effective, (2) basic amino acids were unfavorable, (3) aromatic amino acids were unfavorable, and (4) acidic amino acids were effective. The D-isoglutamine residue of MDP was considered to be essential for the adjuvant activity. The adjuvant activity was decreased by esterification with methanol of the D-glutamic acid residue of MDP and related N-acetylmuramyldipeptides, but the adjuvant activity of D-glutamic acid diamide analogs was similar to that of MDP and its analogs.