for-Met-Leu-Phe-receptor (FPR) is one of the most thoroughly studied neutrophyl receptors. It exhibits seven-transmembrane-spanning segments coupled to the eterotrimeric G- proteins. It has been ascertained that different responses arise from binding of formylpeptide with different isoforms of the specific receptor, which can activate separate second messenger pathways. Several studies have attempted to correlate structure-activity data in order to establish the structural features necessary for biological activity. The goal of studies herein reported has been, instead, to find the role of for-Met-Leu-Phe amide bonds in chemotactic ligand crosslinking. The findings suggest that: a) the amide bond at position 2 must be protic in order to allow the ligand to link and/or to recognize the receptor responsible for eliciting biological activity; b) the amide bond at position 3 surely participates and links the receptor, but its role it is not mandatory; (c) the isoform that elicits chemotaxis is structurally more exacting than the isoform that elicits superoxide anion production.