For-Met-betaAlapsi[CSNH]-Phe-OMe (3), For-Met-betaAlapsi[CH2NH]-Phe-OMe (5), For-Met-NH-pC6H4-SO(2-Phe-OMe 8a), For-Met-NH-mCH4-SO2-Phe-OMe (8b) and the corresponding N-Boc precursors (2, 4, 7a, b) have been synthesized and their activity towards human neutrophils has been evaluated in comparison with that shown by the reference tripeptide For-Met-Leu-Phe-OMe (fMLF-OMe). Chemotaxis, lysozyme release and superoxide anion production have been measured. (1)H NMR titration experiments and IR spectra have been discussed in order to ascertain the preferred solution conformation adopted by the tripeptide 3 with particular reference to the presence of a folded conformation centred at the centrally positioned thionated beta-residue.