The nature of the subunit interactions in the inducible arginine decarboxylase from Escherichia coli B is of considerable interest because of the observed differences in the catalytic activities of the dimer and the decamer; the decamer is active and the dimer is inactive. To study these interactions, inactive dimers were prepared by sodium borohydride reduction of the E-amino--pyridoxal-P Schiff base. Hybrid decamers were then prepared from varying molar ratios of native and reduced dimers. The hybrid decamers were indistinguishable from native decamers as observed in the analytical ultracentrifuge and on acrylamide gel electrophoresis. Kinetic studies indicated that true hybrids were formed rather than mixtures of all-native and all-reduced decamers. Results obtained with the decamers containing 1, 2, 3, or 4 parts in 5 of reduced enzyme showed no significant changes in Km values from the native decamer. However, the Vm values for these hybrids are greater than predicted from the mole fraction of active dimers. For example, the hybrid containing 20% reduced enzyme approaches the Vm of the native decamer. These observations suggest that, in the intact molecule, two active sites cooperate catalytically but only one is catalytically active.