There is much experimental evidence to suggest that eosinophils are important in resistance to parasitic infection. We tested this hypothesis by treating guinea-pigs with rabbit anti-eosinophil serum (AES) and determining the effect of treatment on susceptibility and acquired immunity to Trichostrongylus colubriformis. The treatment markedly reduced the number of eosinophils in peripheral blood and in the small intestine. The number of T. colubriformis present after initial infection and after a second infection was determined in animals treated with AES and in control animals. Administration of AES to guinea-pigs significantly increased the susceptibility of the animals to initial infection with T. colubriformis larvae; the number of worms recovered was nearly doubled. Similarly- administration of AES resulted in a significant diminution of acquired immunity to a secondary infection. These results are consistent with the view that eoxinophils are important in susceptibility and acquired immunity to infection with T. colubriformis in the guinea-pig. Because T. colubriformis infection is confined to the intestinal tract, our results also suggest that eosinophils may be involved in resistance to parasites at the level of the intestinal mucosa.