Serine proteinase inhibitors (serpins) play a vital regulatory role in a wide range of biological processes, and serpins from viruses have been implicated in pathogen evasion of the host defence system. For the first time, we report a functional serpin gene from nematodes that may function in this manner. This gene, named Bm-spn-2, has been isolated from the filarial nematode Brugia malayi, a causative agent of human lymphatic filariasis. Polymerase chain reaction (PCR) and Western blot experiments indicate that Bm-spn-2 is expressed only by microfilariae (Mf), which are the long-lived blood-dwelling larval stage. A survey of the greater than 14,000 expressed sequence tags (ESTs) from B malayi deposited in dbEST shows that greater than 2% of the ESTs sequenced from Mf cDNA libraries correspond to Bm-spn-2. Despite its abundance in the microfilarial stage, Bm-spn-2 has not been found in any other point in the life cycle. The predicted protein encoded by Bm-spn-2 contains 428 amino acids with a putative signal peptide. Antibodies to recombinant Bm-SPN-2 protein react specifically with a 47.5-kD native protein in Mf extract. Bm-SPN-2 is one of the largest of the 93 known serpins, due to a 22 amino acid carboxy-terminal extension, and contains the conserved serpin signature sequence. Outside these regions, levels of homology are low, and only a distant relationship can been seen to a Caenorhabditis elegans serpin. The Bm-spn-2 gene contains 6 introns, 2 of which appear to be shared by both nematode species. The B malayi introns have an extended and conserved 3' splice site and are relatively large compared with C elegans. A panel of mammalian serine proteinases were screened and Bm-SPN-2 protein was found to specifically inhibit enzymatic activity of human neutrophil cathepsin G and human neutrophil elastase, but not a range of other serine proteinases. It is possible that Bm-SPN-2 could function as a stage-specific serpin in the blood environment of the microfilarial parasite in protection from human immunity and thus may be a good candidate for protective vaccine.