Pseudocholinesterase activity of sweat, lacrimal fluid, urine, parotid saliva, whole saliva, whole blood and plasma was estimated using butyrylthiocholine iodide and propionylthiocholine iodide as substrates. There was similarity in activity between erythrocytes and plasma, but considerable differences between whole blood and other body fluids. The level of enzyme activity was approximately the same in sweat, lacrimal fluid, urine, parotid fluid and whole saliva, but it was only about 1/1500 of that of plasma. The enzyme activity in either whole blood or plasma is a good measure of organophosphate exposure but in other fluids activity can hardly give more than an approximate indication of acute poisoning.