An automated method for the determination of bicarbonate in human serum based on the enzymatic reaction between the analyte and phospho(enol)pyruvate (PEP) in the presence of PEP carboxylase is proposed. The analytical reaction is coupled with a derivatization reaction in which the NADH consumed is fluorimetrically monitored (lambda ex = 340 nm, lambda em = 460 nm). A stopped-flow/flow-injection approach is used in which the enzymes (PEP carboxylase and malate dehydrogenase) are immobilized on controlled-pore glass. The linear determination range is between 25 and 300 mmol/l (r2 = 0.9973). The %C.V. for the within- and between-run studies, performed at three concentration levels, ranges between 1.0 and 3.6% and the sampling frequency is 20 per h.