Amphibian, avian and mammal tissues contain a nucleoside phosphotransferase clearly different from those previously described in vegetables and bacteria. Whatever the animal source, the enzyme showed many similar characteristics as far as substrate specificity, dependence upon Mg2+, instability at 37 degrees C, and the protecting effect of nucleotides were concerned. Moreover, when submitted to gel filtration, the enzyme behaved in all cases as a dissociable high molecular weight protein, whose degree of association was controlled by nucleotides. In amphibian and avian tissues multiple forms of the enzyme seem to be present which differ for the substrate concentration at half-maximal velocity (S0.5); the concentration of nucleotide effector which affords half-maximal protection at 37 degrees C (P0.5); and the Hill coefficient for monophosphate donor. Within each single species, the higher the interaction coefficient was, the lower S0.5 and P0.5 values were. In mammalian tissues one form of nucleoside phosphotransferase seems to prevail where cooperative interactions are almost absent and whose S0.5 as well as P0.5 values do not vary significantly from one tissue to another.