Proteases from infective larval (L3) and adult stages of Nippostrongylus brasiliensis were investigated with a combination of techniques involving gelatin degradation and cleavage of fluorogenic substrates. Analysis of L3 excretory-secretory (ES) products revealed enzymes of Mr 51, 58, 79, approximately 150 and approximately 250 kDa. Inhibition profiles indicate that the major 51 kDa protease is a metallo-enzyme. Significantly, little activity was present in larval somatic extracts, suggesting the synthesis of zymogens or precursor forms prior to secretion. Adult ES contained a distinct enzyme, of 50 kDa, and a number of other proteases were detected in somatic extracts of this stage, ranging from 51 to greater than 300 kDa. The largest of these adult somatic enzymes is also a putative metallo-protease. While nearly all enzymes from both L3 and adult are heat labile, incubation at 100 degrees C generated a previously unobserved activity at 20 kDa. Furthermore, a protease of similar size may be found in uninfected rat intestinal tissue, suggesting specific uptake of a host-associated enzyme by the parasite in the form of an inactive, heat-labile complex.