The selectivity of a yeast lactate biosensor with immobilized cells of aerobic yeast Hansenula anomala was studied. Reducing substances potentially present in blood plasma influenced both enzyme and yeast biosensors in the same way; the highest positive error was observed in the case of uric acid. With respect to the metabolic activity of the yeast cells the biosensor was absolutely specific for lactate during the first two weeks; later on the biosensor responded slightly to some other metabolites, especially some sugars and amino acids. Glucose could cause the highest degree of interference, its effect was however completely eliminated by adding sodium fluoride to the reaction solution. The concentration of other metabolites present in blood plasma is not great enough to call a significant positive error. The results thus support the general use of the yeast lactate biosensor for lactate determination in biological material.