Sheep immunized with multiple normal infections of 30,000 Trichostrongylus colubriformis larvae (T.c. L3) suppressed the fecundity, establishment and survival of adoptively transferred adult worms, showing that these parasites were susceptible to the effects of host immunity. When sheep were immunized by four 'truncated' larval infections of 4, 7 or 10 days' (d) duration with 10(5) T.c. L3, animals given 4 x 4d infections were susceptible to challenge, whereas sheep given 4 x 7d and 4 x 10d infections were significantly protected. A serial analysis of the rejection of T. colubriformis from nine sheep given 5 x 7d infections revealed that the challenge larval infection given intraduodenally was expelled within 3 days after challenge (DAC). However, another five of these sheep only rejected around 50% of transferred adult worms by 21 DAC when compared with control animals. The results indicate that stage-specific antigens produced by early L3 and L4 stages of T. colubriformis effectively immunize sheep against a larval challenge but appear less reliably protective against adult worms.