The capability of immunoactive muramyl peptides to bind specifically to rat brain membranes has been discovered. The reaction of an N-acetylglucosaminylmuramyl dipeptide analog having an additional C-terminal lysine residue (GMDP-Lys) with Bolton-Hunter reagent or N-hydroxysuccinimidyl-4-azidosalicylate afforded two acylated derivatives, GMDP-Lys-(Hp) and GMDP-Lys(Azs). Their iodination resulted in radioactive derivatives (spec. act. ∼2000 Ci/mmol) whose binding to rat brain membranes is characterized by Kd∼3 nM and Bmax∼10 fmol/mg membrane protein. Binding could be inhibited by muramyl dipeptide (MDP), GMDP-Lys, and GMDP, while fragments of the latter, N-acetylglucosamine, dipeptide and disaccharide, were ineffective. © 1989.