The adrenergic mouse neuroblastoma clone NIE 115 contains two species of AChE which sediment at 4S and 11S. These two species are found both in logarithmic growing phase cells (round neuroblast morphology) and in cells which have undergone morphological differentiation due to the elimination of serum. The 4S form is predominant in the growing cells (70 per cent) whereas the 11S form is more abundant (55 per cent to 65 per cent) in the cells with neurites. When protein synthesis is inhibited by cycloheximide, there is an increase states : we postulate that the 11S species is formed by a conversion of the 4S species. When cell division is blocked by sodium butyrate, there is an increase in AChE activity but no modification in the proportion of 4S/11S species as compared to the cells in growing phase. We were unable to associate temporally the increase in the 11S species with neurite outgrowth ; when cells were allowed to retract their neurites and were subsequently maintained in a GROWING state for 10 to 15 days, the 11s species still predominates. Although the presence of the 11S molecule cannot always be correlated with the state of morphological differentiation, the shift in sedimentation coefficient of AChE form 4S to 11S might eventually constitute a reference in the study of biochemical events leading to terminal differentiation in this system.