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A comparative study has been carried out on effects of berberine (diisoquinoline alkaloid) and sanguinarine and chelidonine (benzophenanthridine alkaloids( on erythrocyte acetylcholinesterase and serum butyrylcholinesterase from human blood. The studied alkaloids have been shown to be strong reversible inhibitors of the cholinesterase activity. Acetylcholinesterase is more sensitive to their action, than butyrylcholinesterase. The type of reversible inhibition was determined, and inhibitor constants were calculated. It is revealed that the character of inhibition is identical for the both cholinesterases. Berberine and sanguinarine are competitive-noncompetitive inhibitors, whereas chelidonine, a competitive inhibitor.