SummaryThe effect of host serum onin vitrofeeding byTrichostrongylus colubriformiswas studied by incubating adult helminths in goat serum containing the dye, Rhodamine B. The amount of dye ingested was determined by fluorometric analysis. Immune serum from goats infected withT. colubriformissuppressed helminth feeding, while normal serum from uninfected goats did not. Suppression of feeding by immune serum increased with the duration of the host's infection. Heat‐inactivation (56°C) of immune serum did not affect its suppressive activity. Pre‐exposure of worms to immune serum decreased subsequent feeding activity. However, rigorous washing of helminths restored their feeding to levels that were similar to untreated worms. Indirect immunofluorescent studies with immune serum and FITC conjugated rabbit anti‐goat IgG demonstrated binding of immunoglobulin to the cuticle, stoma and excretory pore of whole worms. Feeding inhibition of immune serum was associated with IgG1isotype. Results of the present studies indicated that IgG was responsible forin vitrosuppression ofT. colubriformisfeeding and may be one effector of immunity toT. colubriformisin th