The enzyme catalysing the polyamine-stimulated modification of Physarum ornithine decarboxylase in vivo was partially purified and its activity on purified ornithine decarboxylase was examined with respect to its specificity for various amines. Spermidine, spermine and several polyamine analogues strongly promoted this reaction in vitro (apparent Km in the 0.1--0.5 mM range), whereas putrescine (apparent Km 5.33 mM) and several related diamines were not nearly as effective. In agreement with this, sensitivity studies performed in vivo also suggested that cellular spermidine, and not putrescine, is critical in modulating ornithine decarboxylase activity by this post-translational control. Unlike putrescine, or other diamines, 1,3-diaminopropane demonstrated a functional similarity to the polyamines in stimulating this reaction. This study has demonstrated a method whereby non-physiological amines capable of depressing ornithine decarboxylase activity by this natural feedback mechanism can be readily identified for further evaluation of their potential use in the experimental and medical control of polyamine biosynthesis.