The activities, substrate specificities, and reactivities with third compounds were tested with serum cholinesterases obtained from 32 warm-blooded animal species, some of which were being investigated for the first time. According to the enzyme properties a classification of the animals is attempted. The first group, consisting of a horse, donkey, mule, and zebra, was found to be entirely different from the ruminants. The rodents exhibited a rather diversified type of serum cholinesterase. The two kangaroo species examined were found to be unique with regard to a relatively strong inhibition by butanol, and the unexpected activation by choline was a prominent characteristic of many bird serum cholinesterases.