The effect of one or more immunising infections on the susceptibility of mice to a subsequent challenge dose of Nematospiroides dubius larvae has been investigated. Following a single infection, animals developed a partial resistance characterized by their capacity to kill no more than half of the challenge dose of larvae. This low degree of immunity prevailed for some weeks after initial infection. A second immunising infection was found to induce a sudden increase in resistance such that at least 95% of a subsequent challenge dose of larvae was killed. This increase occurred 6 to 8 days after the second immunisation. Similar results were observed for mice immunised orally or intravenously. During the course of immunisations, absolute and differential cell levels in the blood and peritoneum as well as serum concentrations of the various immunoglobulin classes were determined. The results are considered in terms of the possible involvement of two distinct mechanisms of immunity to N. dubius, one requiring the participation of macrophages and the other, eosinophils.