A bioassay for the measurement of an ovine eosinophil differentiation factor (EDF) was developed. Recombinant murine interleukin-5 (rmIL-5) and supernatant from concanavalin A stimulated lymphocytes induced eosinophil differentiation and proliferation in ovine bone marrow cell culture, which was determined by an eosinophil peroxidase assay (EPO). The assay was used to measure the production of an ovine eosinophil differentiation factor by lymphocytes following experimental infection of sheep with Trichostrongylus colubriformis. Sixteen sheep were immunised and challenged with T. colubriformis and grouped into responders (n = 10) and non-responders (n = 6) based on the lower 99% confidence limit of the worm burden of the unimmunised control group (n = 10). Total worm burdens of all animals were established 10 days after initial challenge. Peripheral blood lymphocytes at Day 0 (pre-challenge values) and Day 9 (post-challenge values) from all sheep and lymphocytes of mesenteric lymph nodes from eight sheep were stimulated with third stage larval antigen of T. colubriformis. The resulting lymphocyte conditioned medium (LyCM) was tested for EDF by its ability to induce growth and differentiation of eosinophils in liquid bone marrow cell cultures. No significant difference was found between the animal groups. No correlation was found between EDF activity and total worm numbers.