The interaction of arenicin-1, an antimicrobial peptide from the lugworm Arenicola marina with the protein C1q of the human complement system has been analyzed using enzyme-linked receptor sorbent assay and ELISA. Arenicin-1 and C1q were shown to form a stable complex that persisted at elevated ionic strength (0.5 M NaCl). The ability of arenicin-1 to interact with C1q is comparable to that of the porcine cathelicidin protegrin-1, an antimicrobial peptide that has a spatial structure similar to that of arenicin (an antiparallel β-hairpin stabilized by disulfide bridges).