Wistar-Furth strain male rats and CFW strain male mice were immunized against Trichinella spiralis using an antigenic fraction derived from a cell-free homogenate of mature muscle larvae. In rats, animals immunized with 250 mug of antigen harboured significantly fewer (135000) muscle larvae 30 days after oral challenge than controls (231000). Further 7-day-old adult worms derived from immunized rats shed 48% fewer (P less than 0.001) newborn larvae over a 24h period in vitro than adult worms from non-immunized animals. Mice were injected with either 10 or 100 mug of antigen. In comparison with non-immunized controls, mice immunized with 100 mug of antigen harboured significatnly fewer adult worms at days 7 and 9 after oral challenge infection, while female worms recovered from immune mice on days 6-10 after challenge shed fewer newborn larvae in vitro. Finally, mice immunized with 100 mjg of antigen harboured significantly fewer (10391) muscle larvae at 30 days after challenge than did controls (47750). Immunization of mice with 10 mug of antigen did not induce a statistically significant reduction in adult worms at either day 7 or 9 after challenge (P less than 0.5). However, adult female worms from mice receiving 10 mug of antigen still shed fewer larvae than did adults from control mice (P less than 0.05). Mice immunized with 10 mug of antigen harboured significantly fewer (13700) recoverable muscle larvae than did controls at 30 days after challenge (39000).