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Change in b-tubulin nucleic acid and protein sequences was the only known difference between Haemonchus contortus fenbendazole (FBZ)-resistant and -susceptible isolates. This change was sufficient to determine the pathologic effect induced by FBZ treatment. This research was initiated to investigate further differences from these two isolates. Since b-tubulin is involved in formation of microtubule, which has functions in secretory vesicle transport, DNase activities from excretory/secretory products (ESP) of the two isolates were compared, based on pH, sensitivity to DNase inhibitors, molecular masses and production of 3'-OH. The most significant difference detected was that a 38.5 kDa DNase activity was identified from ESP of H. contortus FBZ-susceptible isolates but not from those of H. contortus FBZ-resistant isolates. However, it was shown that the 38.5 kDa DNase is expressed with similar level of activity in intestine and whole worm of H. contortus FBZ-resistant and -susceptible isolates. This result demonstrated that the secretory transport pathway of the 38.5 kDa DNase was inhibited by unknown mechanisms, which may be related with b-tubulin sequence change in FBZ-resistant isolates. Other DNases of 34, 36 and 37 kDa were detected from ESP of both H. contortus FBZ-resistant and -susceptible isolates. Overall DNase activities found from ESP of these two isolates were not inhibited by 10 mM EDTA at pH 5.0, but largely inhibited by pH 7.0. In addition, DNase activities in two isolates produced DNA fragments with mixtures of 3'-hydroxyls (OH) and 3'-phosphates (P) at each pH although the 3'-end labeling ratios at pH 5.0 and 7.0 were shown different. Identification of inhibition of the 38.5 kDa DNase secretion in FBZ-resistant isolates suggests existence of further differences, in addition to b-tubulin sequence change, in two isolates. This shows complex effect of FBZ on H. contortus biological mechanisms.