The active sites of acetylcholinesterase multiple forms from four widely different zoological species (Electrophorus, Torpedo, rat and chicken) were titrated using a stable, irreversible phosphorylating inhibitor (O-ethyl-S2-diisopropylaminoethyl methyl-phosphonothionate). In all cases, we found that within a given species, the molecular forms we examined were equivalent in their catalytic activity per active site. As pure preparations of the molecular forms of Electrophorus acetylcholinesterase were available, we were able to establish that one inhibitor molecule binds per monomer unit for each of them. This had already been shown by several authors for the tetrameric globular form, but not for the tailed molecules. Analysis of the phosphorylation reaction showed that they are equally reactive. Under our experimental conditions, their turnover number per site was 4.4 x 10(7) mol of acetylthiocholine hydrolysed . h-1 at 28 degrees C, pH 7.0. The corresponding value was less than half for Torpedo (1.64 x 10(7) mol . h-1), and again lower for rat (1.32 x 10(7) mol . h-1) and chicken (1.05 x 10(7) mol . h-1). In the case of rat acetylcholinesterase, the activity per active site of solubilized (with or without Triton X-100) and membrane-bound enzyme were identical. We discuss the implications of these findings with respect to the quaternary structure of acetylcholinesterase, and to the physico-chemical state and physiological properties of its molecular forms.