Context: Erythrocyte membranes regulate many enzyme activities, including carbonic anhydrase II (CA II). Membrane fluidity is associated with alterations in protein function and protein-protein interactions. Objective: The purpose of this study was to show the human CA II (hCA II) activity regulation by human erythrocyte membranes from diabetic and hypercholesterolemic subjects. Materials and methods: Erythrocyte membranes were obtained from diabetic, hypercholesterolemic, and healthy subjects. hCA II activity was measured using the electrometric method. Results: hCA II activity was increased in vitro by membranes from both diabetic and hypercholesterolemic patients, with hypercholesterolemic membranes exhibiting a greater increase. Conclusion: Changes in membrane composition may affect the erythrocyte membranes' capacity to increase in vitro hCA II activity.