The diamine putrescine and the polyamines spermidine and spermine which are derived from it have been implicated in the regulation of growth processes. 1 In attempts to delineate the still controversial roles of these bioamines, several reversible inhibitors of the pyridoxal phosphate (PLP)-dependent enzyme L-ornithine carboxylyase (ODC, EC 4.1. 1.17) which catalyzes the conversion of L-ornithine to putrescine, have been prepared. 2 A new and elegant approach to specific, irreversible enzyme inactivation is to design inhibitors possessing latent reactive groupings which are unmasked at the enzyme’s active site as a result of the normal catalytic turnover. 3 Such known inhibitors are analogues of the normal enzyme substrate, but, less obviously, in view of the microscopic reversibility principle, they may conceptually be analogues of the product. This communication discloses that, not only the ornithine analogues 1, but also the putrescine analogues 5-hexyne-1, 4-diamine (2) and íraní-hex-2-en-5-yne-1, 4-diamine (3) are irreversible inactivators of ODC, and that, ineach case, the mechanism of inhibition demands activation of the inhibitor by the target enzyme. Incubation of the enzyme preparation, obtained from the