Genes encoding antigens of adult worm, newborn larvae and muscle larvae of Trichinella spiralis were identified by immunoscreening their corresponding cDNA libraries. The cDNA libraries of T. spiralis adult (3 day old, Ad3) and newborn larvae (NBL) were screened using the serum of a pig infected with 20,000 muscle larvae (ML) at 26 days post-infection (dpi). Fifty-two positive clones representing 18 unique genes were obtained from the Ad3 cDNA library. The deduced amino acid sequences of 8 cDNAs were sequence homologues of the serine protease-like protein family. In the screening of NBL cDNA library, 81 positive clones representing 8 unique genes were isolated and of these, 70 clones corresponded to an NBL stage-specific serine protease gene. The ML cDNA library was screened using pig anti-Trichinella serum obtained at 60 dpi and 18 positive clones representing 8 unique genes were identified. Ten clones encoded a protein that is identical to a T. spiralis serine protease inhibitor. In general, our results revealed that antigenic serine protease-like proteins were found during the two invasive stages, Ad and NBL when these libraries were screened using 26 dpi antiserum, whereas a serine protease inhibitor was found in abundance in the ML library when it is screened using 60 dpi antiserum. These data are consistent with serine proteases playing an important role during the invasive stages of Trichinella infections, but become inhibited or internally controlled when the parasite enters a more stable, non-developing environment.