Thiabendazole inhibits nematode development before the third stage is reached. The minimum effective concentration is 0.1 microgram/ml during the first 48 hours of development and 1.0 microgram/ml at 72 hours of development. The drug has no effect on the third stage but is again effective on larvae that have undergone 72 hours of development in the intestine of the specific host (dog). Increasing concentrations of the drug seem to be necessary to kill the larvae as the organism progresses in development. Concentrations of the drug that kill the organism before it enters the third stage (infective form) are exceeded in the blood and skin of sheep by a single 50 mg/kg oral dose. It is speculated that the drug is not effective against the infective stage because it does not enter the larvae and that the larvae progress in the skin of man to a stage where they ingest the drug or become permeable to it.