We employ a new carbonate ion selective membrane electrode in a continuous flow procedure for the direct determination of the CO2content in human serum samples. The method yields excellent precision and accuracy with a standard deviation of a single measurement (on a day-to-day basis) of 1.2 meq/liter. Comparison of our analyses on hospital samples with independent analyses using classical methods gave good agreement and indicates that the proposed electrode method will be useful for routine clinical analyses with considerable reduction in sample handling and cost.