Phosphoenolpyruvate carboxylase (PEPC) purified from maize (Zea mays L.) leaves associates with maize leaf chloroplast membrane in vitro. The binding of PEPC to the membrane results in enzyme inactivation. A protein isolated from a maize leaf chloroplast membrane preparation inactivated PEPC. Treatment with membrane preparation or with partially purified inactivating protein accelerates PEPC inactivation at low temperature (4°C). Interaction of PEPC with chloroplast membrane or inactivating protein may inactivate the enzyme by influencing dissociation of the enzyme active tetramer.