SummaryThis paper describes experiments which demonstrated that the survival of Nematospiroides dubius was severely impaired in mice treated with immune serum. CFLP donor mice were given a series of infections ranging from 25 to 200 infective larvae, at weekly intervals for 6 weeks. The mice were treated with anthelmintic on day 21 and/or day 28 to prevent the accumulation of lethal numbers of parasites in the intestine, and were bled between day 42 and day 49. Female NIH recipient mice were given a total of 2.0–2.5 ml of immune serum i.p., in several separate smaller doses at various times in relation to the day of infection. Between the administration of immune serum begun during the first 4 days of infection and the animals being killed within the next 3 weeks, the mice harboured fewer worms than control animals, the worms were stunted and their fecundity was greatly reduced. Furthermore, these worms were subsequently lost from the intestines of treated mice, during and after the fourth week of infection. These effects on N. dubius were not observed when mice were given normal serum nor when immune serum was administered after day 6 of infection. The delayed rejection of adult worms from mice treated with immune serum is of particular significance and suggests that immune serum contained factors which facilitated the expression of a second component in worm expulsion not normally effective in a primary infection. The possible immunological mechanisms underlying these findings are discussed and related to the immunosuppression which N. dubius is known to induce in the hos