Cyclophosphamide (CP; 2.5-10.0 mM) reversibly inhibits acetylcholinesterase (AChE) activity of chicken brain (58-84%) in a concentration-dependent manner, the IC50 (the concentration causing 50% inhibition) being about 2.0 mM. Lineweaver-Burk plots indicated that the nature of this inhibition is mixed; intercept and slope replots are straight lines, showing that the inhibition is complete rather than partial. The value of Km for AChE is found to be 108 microM, while values of KI and Ki are found to be 1.5 and 5.1 mM, respectively, Ki is greater than KI indicating that uncompetitive inhibition is predominant over noncompetitive inhibition.