Plasma cholinesterase activity levels of various species may be of interest to toxicologists or pathologists working with chemicals that interfere with the activity of plasma cholinesterase.We used a pH titration method to measure the plasma cholinesterase activity of six mammalian species.Plasma cholinesterase activity varied up to 50-fold between species: sheep (88±45nM acetylcholine degraded per ml of test plasma per minute), cattle (94±35), western grey kangaroos (126±92), alpaca (364±70), rats (390±118) and horses (4539±721).We present a simple, effective technique for the assay of plasma cholinesterase activity levels from a range of species. Although labour-intensive, it requires only basic laboratory equipment.