Two effectors of ornithine decarboxylase (ODC; L-ornithine carboxy-lyase, EC 4.1.1.17) have been extracted from an ODC- (speC-) mutant, Escherichia coli MA 255. One of these is an ODC inhibitor (Mr 15,000 +/- 2000) that is labile to trypsin; its activity increases 20-fold in response to increased polyamine levels in the growth medium. It has additional characteristics similar to those of the ODC antizyme of eukaryote cells: it is a noncompetitive inhibitor of ODC; the complex formed between ODC and the ODC inhibitor can be dissociated with salt to provide active ODC and active ODC inhibitor; furthermore, this E. coli ODC inhibitor is inhibitory to eukaryote ODC. A thermostable nondialyzable factor that activates ODC in vitro has also been extracted from MA255; increased polyamine levels in the growth medium caused a 1.6-fold increase in the activity of this ODC activator. Effectors with comparable activities have also been identified in the parent ODC+ (speC+) strain MA197. The fluctuations of the intracellular levels of these two ODC effectors during the growth of E. coli MA255 have been related to the temporal changes of the activity of ODC in the parent ODC+ MA197 strain. The mode of interaction of these three macromolecules, as reflected in the changes of the activity of ODC, appears to be complex. The results suggest that ODC activity may be controlled post-translationally by macromolecules that act as positive and negative effectors and whose levels fluctuate in response to the concentration of the end products of the reaction.