Abstract: The present invention relates to an agent for preventing and treating thrombocytopenia which comprises, an an active ingredient, a muramyldipeptide derivative of the formula: ##STR1## wherein X represents an amino acid residue selected from L-alanine, L-serine and L-valine, Y represents ##STR2## wherein R.sub.1 represents a carboxyl group, n represents an integer of 1 to 6, A represents a saturated aliphatic hydrocarbon group having 7 to 29 carbon atoms which may have branched chains, and Acyl represents an acyl group having 2 to 6 carbon atoms, and the agent exhibits excellent preventing and treating effects on thrombocytopenia when administered orally or parenterally.