The transmembrane glycoprotein gp41 of HIV-1 contains a C1q binding domain (HIVenv 583-610) and activates the human complement system through the classical pathway. Based on structural and functional similarities between human defensins (human neutrophil peptide, HNP 1-3) and synthetic peptides representing the env 583-610 region of HIV-1, we found it interesting to investigate the C1q binding and complement activating ability of human defensins. Human defensins were purified and characterized by size exclusion chromatography, ultrafiltration, gel electrophoresis and HPLC. The complement activating ability of the purified peptides was assessed in a solid-phase immunoassay. Defensins, fixed to an ELISA plate, were able to bind the C1q subcomponent of the first complement component (C1), triggering the classical pathway of complement activation which led to C4b binding to the plate. Reduction and subsequent alkylation of disulfide bridges of defensins greatly decreased the C1q binding ability but complement activation (C4b binding) remained high. Further acetylation of the reduced defensin peptide resulted in a molecule which bound very little or no C1q but still activated the complement cascade. These phenomena indicate that defensins interact with the complement system via C1q-dependent and C1q-independent mechanisms, and extend the number of functional similarities between defensins and gp41 of HIV-1 to include C1q binding and complement activation.