Simultaneous immunological titration of DOPA and 5-hydroxytryptophan decarboxylase activities, from a number of tissues of various species, showed that the two activities were not distinguishable with a monospecific antiserum to hog kidney decarboxylase. Together with previous findings, these data firmly establish the concept that in mammalian tissues the two enzyme activities are associated with a single protein, namely aromatic L-amino acid decarboxylase.