A panel of conformationally constrained, decapeptide agonists corresponding to the C-terminal "effector" region of human C5a (C5a65-74 or ISHKDMQLGR) was evaluated for the ability to increase vascular permeability. One constrained analog, acyl-YSFKPMPLaR, expressed between 2 and 10% of full C5a activity in increasing vascular permeability, as measured by the extravasation of Evans blue dye in guinea pig skin. This analog was at least 10-fold more potent than its unconstrained sister analog C5a65-74465, F67++ (YSFKDMQLGR), which was used as an internal standard in these assays. Neither acyl-YSFKPMPLaR nor YSFKDMQLGR changed the transvascular equillibrium of an electrolyte, 86Rb, at the peptide injection site. However, both peptides effected a significant increase in the extravasation of two macromolecules, 125I-labeled bovine serum albumin and 131I-labeled monoclonal antibody BL-3. The extravasation of Evans blue dye mediated by 0.03 to 0.1 nmol of acyl-YSFKPMPLaR was nearly abolished by 1 to 10 nmol of the antihistamine diphenhydramine. For YSFKDMQLGR, however, the sensitivity toward diphenhydramine was observed only at low concentrations of the peptide (1 nmol). When incubated in human and mouse sera, acyl-YSFKPMPLaR was shown to be stable toward the actions of serum carboxypeptidases. However, the unconstrained analog YSFKDMQLGR was rapidly converted to the des-Arg form under the same conditions. Taken together, these results support a growing body of evidence that unique topochemical features expressed in conformationally constrained agonist analogs of C5a contribute favorably to their ability to modulate vascular permeability and to their stability in serum.