Both eosinophil chemotactic factor (ECF) and neutrophil chemotactic factor (NCF) activities were demonstrated in excretory/secretory (ES) products and homogenates of Haemonchus contortus and Teladorsagia circumcincta larvae and adult worms in a modified checkerboard assay using a micro-chemotaxis chamber. Neutrophil chemotaxis was seen in 28 of 35 experiments and eosinophil chemotaxis in 20 of 38 experiments. Chemokinetic activity for neutrophils and eosinophils (accounting for 40-50% of total cell migration) was also apparent in only three parasite products for each cell type. Significant NCF activity was present in six of seven adult worm ES products (three of four from T. circumcincta and in all three from H. contortus) and ECF activity in four of five adult ES products, whereas fewer L3 incubates, particularly of T. circumcincta, contained chemotactic activity. All parasite homogenates, with one exception for ECF, were chemotactic for both neutrophils and eosinophils. The sequential use of cellulose ultrafiltration membranes of decreasing pore size did not identify precisely the molecular weight of the NCF and ECF but indicated that the active chemicals were greater than 10 kDa and probably greater than 30 kDa.