Human neutrophil peptide-1 (HNP-1) is a member of the alpha-defensin family. Defensins are cationic antimicrobial peptides, which play an important role in the antimicrobial response to microorganisms. In addition, recent studies have revealed the involvement of defensins in inflammation, immunity and wound repair. Defensins are present in the azurophilic granules of neutrophils and are released upon neutrophil stimulation. Previous studies showed that HNP-1 binds to C1q and inhibits the classical complement pathway. In view of the structural and functional similarity between C1q and MBL, we have now examined the interactions between HNP-1 and MBL. We observed a dose-dependent binding of HNP-1 to MBL in calcium-free buffer, indicating that HNP-1 binds to MBL most likely via the collagenous domains. To identify the binding sites in HNP-1 involved in the binding to C1q and MBL, we used a series of overlapping synthetic linear peptides that spanned the entire HNP-1 sequence. Both MBL and C1q showed a dose-dependent binding to the same set of peptides, suggesting a similar binding site in HNP-1 for both MBL and C1q. Strongest binding was observed to peptides containing the C- or N-terminal part of the HNP-1 molecule. Using an ELISA based system, we demonstrated that HNP-1 inhibits activation of both the classical pathway and lectin pathway of complement. Furthermore, we demonstrated that C1q and MBL can form complexes with HNP-1 in solution. Together, the data indicate that HNP-1 interacts with both C1q and MBL efficiently resulting in inhibition of both the classical and the lectin pathway of complement. We conclude that HNP-1 may play a role in protection against tissue injury during inflammatory conditions by inhibiting the early phase of complement activation.