In rat pheochromocytoma (PC12) cells treated with nerve growth factor (NGF), there are several molecular forms of the enzyme acetylcholinesterase (AChE) which sediment on sucrose density gradients at 4 to 6, 10, and 16 S, respectively. We have investigated the cellular localization of these forms in PC12 cells. In order to determine which forms are soluble and which are membrane bound, we extracted PC12 cells in buffers of various ionic strengths and detergent compositions. To distinguish internal from external forms of the enzyme, we examined the effect of di-isopropyl fluorophosphate and BW284c51 dibromide, membrane-permeable and -impermeable inhibitors of AChE, respectively, AChE forms in intact cells. We also determined the susceptibility of the forms in intact cells to collagenase treatment. Based on these studies, we conclude that the globular G1 and G2 (4 to 6 S) forms are internal and consist of both soluble and membrane- associated species. Thirty percent of the G4 (10 S) form is bound to cytoplasmic membrane structures, while the remainder occurs as an integral component of the plasma membrane. The asymmetric A12 (16 S) form is also a surface protein but is extracted by high salt without detergent and is released from intact cells by collagenase. This form thus contains a collagenous domain and is located outside of the plasma membrane, where it may be associated with an extracellular matrix.