Abstract: A method for treating and preventing bacterial infections which comprises administering a 6-O-acylmuramyl depeptide of the formula: ##STR1## wherein X represents a divalent residual group of an amino acid such as L-alanine, L-serine, L-valine, etc., Y represents a residual group of a middle to higher fatty acid having 10 to 60 carbon atoms and isoGln represents a residual group of isoglutamine, or a pharmaceutical composition containing a 6-O-acylmuramyl dipeptide of the formula(I) as an active ingredient to a patient afflicted therewith.

Abstract: Muramyldipeptide derivatives of the formula (I): ##STR1## wherein X represents an amino acid residue such as of L-alanine, L-serine, L-valine, glycine, etc., and Y represents a group ##STR2## wherein R.sub.1 represents a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a carboxamide group or a carboxyl group, n represents an integer of 1 to 6, A represents a straight or branched chain, saturated or unsaturated aliphatic hydrocarbon residue of 7 to 30 carbon atoms, and "Acyl" means an acyl group of an aliphatic carboxylic acid having 2 to 6 carbon atoms. These compounds have excellent adjuvant activity and/or prophylactic and therapeutic effects against microbial infections.