Certain formyl peptides are powerful chemoattractants towards neutrophils. In this study, several formyl tripeptides were synthesized and used to investigate the effects of different amino acid residues in position 1 on their ability to stimulate neutrophil chemotaxis. Pig neutrophil chemotaxis towards the formyl tripeptide, HCO-Ac(3)C-Leu-Phe-OMe 1, where Ac(3)C represents 1-amino-1-cyclopropane carboxylic acid, was observed. Pig neutrophil chemotaxis towards a very similar formyl tripeptide, HCO-Aib-Leu-Phe-OMe 2, where Aib represents alpha-amino isobutyric acid, was not observed. Compared to the isopropyl group, it was shown that the cyclopropyl group induces a greater percentage of the E conformation about the formamide functionality in these peptides. For 1 and 2, the E isomer distributions in CDCl3 are 36 and 9%, respectively. Since a major difference between these two peptides is the Z/E isomeric distribution, one implication is that the peptide-receptor site interactions involving the E conformer are more effective than those of the Z conformer. No pig neutrophil chemotaxis towards the formyl tripeptides, HCO-Ala-Leu-Phe-OMe 3 and HCO-Gly-Leu-Phe-OMe 4 was observed. These formyl tripeptides exhibit a low percentage of the E isomer, similar to that of peptide 2.