The tissue-invasive infective and lung-stage larvae of the nematode Ascaris suum were found to release proteinases during culture in vitro. This activity contained multiple proteolytic enzyme activities, as defined by pH optima, substrate specificities, and inhibitor profiles. Chymotryptic, tryptic collagenolytic and elastolytic activities were produced by both developmental stages, with major pH optima at pH 6 and 9, and there were indications of unusual interactions between the enzymes. The set of proteinases released was found to be specific to each stage of the parasite, although these included some activities which were indistinguishable between the products of the two. The in vitro-released materials of the tissue-parasitic stages of Ascaris are already known to be potently antigenic. Here, we found that this antigenicity was reflected by inhibition of the proteinases of both stages by serum antibody from infected animals. This inactivation of major secreted enzymes of this parasite could presumably contribute to impairment of survival and migratory potential in sensitised hosts.