A new in vitro assay of benzimidazole activity against adult Oesophagostomum dentatum is described. The method is based on the ability of O. dentatum to migrate through polyamide nets after exposure to various concentrations of benzimidazole. To determine an appropriate mesh size, control worms and worms exposed to 10 microM oxfendazole for 24 h were allowed to migrate through nets with various mesh sizes (300-500 microns) for up to 1 h. A mesh size of 350 microns and migration periods of 10, 20 and 30 min were selected. Exposure to oxfendazole at 10 microM for 24, 48 and 72 h inhibited the migration in a time-dependent manner. After 72 h of exposure and with a 20-min migration period, the EC50 of oxfendazole for O. dentatum was 0.564 microM. In further studies the activities of albendazole sulphoxide, albendazole, cambendazole, fenbendazole, flubendazole, luxabendazole, mebendazole, oxfendazole, oxibendazole, parbendazole and thiabendazole were compared. The worms were exposed to each drug at two concentrations (0.1 and 3.16 microM) for 72 h. At 3.16 microM there were no significant differences in the activity of the drugs. At 0.1 microM significant differences in activity were found. Albendazole sulphoxide and oxfendazole were poor inhibitors of migration compared with their parent compounds, albendazole and fenbendazole.