Several indices of the immune response to Nematodirus battus infection in lambs offered differing levels of dietary protein were quantified. Lambs were offered either a complete basal ruminant diet (13.2% crude protein (CP)) or the same diet supplemented with fish meal as a source of rumen bypass protein (18.3% CP). Lambs from each dietary treatment group were given either a 7-week continuous trickle infection with N. battus L3 or remained uninfected. All lambs were drenched with anthelmintic at week 8 post-infection (PI), challenged with a single dose of 30,000 N. battus L3 1 week later, and killed 9 days post-challenge (PC). Previous infection induced a significant reduction in worm burdens (p < 0.001) and enhancement of immune responses when compared to challenge controls. Among previously infected lambs, protein supplementation did not reduce worm burdens significantly, although there was a trend for fewer worms in the supplemented lambs. However, a significant increase in mucosal globule leucocyte (p < 0.05) and eosinophil (p < 0.05) numbers was evident. Supplementation (p < 0.05) and previous infection (p < 0.001) both enhanced serum anti-worm IgG titres over time. Peripheral blood eosinophil counts were not affected by supplementation but were significantly elevated over time as a result of previous infection (p < 0.001). Since there were no significant differences in worm burdens of supplemented and unsupplemented previously infected lambs, it was of interest to determine whether lambs possessed an innate ability to regulate their parasite burden. Hence they were re-grouped based on an arbitrary cut-off burden of 1000 worms. High responders (HR) had burdens below 1000 worms, while low responders (LR) had burdens above this value and challenge controls were pooled. The data were re-analyzed based on these groupings and showed significant reduction in worm burdens between all three groups (p < 0.001). Globule leucocytes were the only cell type that appeared to be significantly more abundant in the intestinal mucosa of HR (p < 0.001). Serum antibody responses (p < 0.05) and peripheral blood eosinophil counts (p < 0.01) were significantly elevated over time in accord with the degree of responsiveness. The results of this study suggest that supplementation of protein upon an adequate basal diet of lambs previously exposed to N. battus does not significantly enhance worm regulation despite significant increases in cellular and antibody responses. The immunity acquired is characterized by reduction in worm burdens, elevated anti-worm antibodies and a cellular inflammatory response. The identification of HR and LR essentially shows that when the protein supply is adequate, the predominant host effect influencing the pathogenicity of the parasites is the level of genetically determined susceptibility of the host.