ELISA assay showed that sera from each of 729 healthy donors contained antibodies to the minimal component of bacterial cell walls, N-acetylglucosaminyl-N-acetylmuramyt dipeptide (GMDP). Anti-GMDP antibody levels were determined for 686 sera, which were classified into 3 groups: high- (17.6%), medium- (68.7%) and low-responder (13.7%). Inhibition analysis performed on representative sera showed that a proportion contained specific anti-GMDP antibodies reacting only with GMDP (i.e., GMDP interaction with anti-GMDP antibodies was inhibited by GMDP only) whereas the remaining sera reacted both with GMDP and with tetrasaccharide (GlcNAc-MurNAc)(2) (i.e., GMDP interaction with anti-GMDP antibodies in the latter sera was inhibited by both GMDP and tetrasaccharide). Inhibition analysis indicated, moreover, that the anti-GMDP antibodies contained in high-responder sera had higher affinity than those present in low-responder ones: GMDP inhibited the GMDP + anti-GMDP antibody interaction by 88.7% in the former sera vs. 53% in the latter. Sera contained both IgM and IgG antibodies to GMDP, but the mean level of anti-GMDP IgG antibodies in the high-responder sera was 30 times higher than in the low-responder ones.