To evaluate their protective potential, excretions and secretions of adult female Angiostrongylus cantonensis were prepared from an in vitro culture fluid and injected together with Freund's complete and incomplete adjuvants into rats and mice. Under appropriate conditions, judging from reduced mortality rates and/or prolonged survival periods, these animals were protected against a lethal challenge infection with 3rd-stage larvae. Although both ‘immunized’ and unimmunized mice died following such a lethal infection, the survival period for the ‘immunized’ group was significantly longer than that of unimmunized controls (40·8 compared with 26·7 days). On the other hand, the mortality rate of ‘immunized’ rats was reduced to 25% as compared with 100% in unimmunized controls. There was also evidence of adverse effects on parasite development in these ‘immunized’ rats. The number of young adult worms recovered from the brain 21 days after challenge infection and both the number and size of mature male and female worms in the lungs 60 days after infection were significantly reduced in the ‘immunized’ rats. Moreover, the reproductive activity of female worms was depressed, as evident from the output of 1st-stage larvae in the faeces and the number of eggs released during maintenance in vitro. The quantity of proteins released during in vitro maintenance of adult worms in a chemically defined serum-free medium was 0·2–0·5 μ/worm/day. The concentrated culture fluid contained at least 7 different antigenic components when analysed with hyperimmune rabbit antiserum to female excretions and secretions (FES) antigen. Polyacrylamidegelelectrophoresis in sodium dodecylsulphate showed the presence of 3 major and at least 12 minor protein components with molecular weights varying from 24000 to 115000 daltons. Different lines of evidence presented in this study show immunological cross-reactivity among antigens from different developmental stages.