We measured the levels of lysosomal enzymes and acetylcholine in Wuchereria bancrofti-infected asymptomatic microfilaraemic human serum, and found a significant decrease in the activity of beta-glucuronidase and acid phosphatase compared to normal serum. Acetylcholine levels were also decreased during infection. However, after giving diethylcarbamazine (6 mg/kg body wt/day) the level of lysosomal enzymes and acetylcholine increased and reached a normal value after two weeks of therapy. It is proposed that parasites secrete acetylcholinesterase in the circulation which degrades acetylcholine. Since acetylcholine stimulates the release of lysosomal enzymes and phagocytosis, the immune response of the host is suppressed during infection. During diethylcarbamazine (DEC) therapy the parasitic enzyme is inhibited by the drug and the normal level of acetylcholine is resumed, which again stimulates the release of lysosomal enzyme and the process of phagocytosis.