Lactate (lactic acid) is an end product of glycolysis and cell anaerobic metabolism indicator. Reducing oxygen delivery to the cells leads to increased lactate production and blood concentration. Blood lactate concentration is usually measured to monitor tissue hypoxia and to assess oxygen transport efficacy in critically ill patients (with severe pneumonia, acute respiratory distress - syndrome, shock, etc.) Change in the blood lactate level could also indicate training intensity and post - exertional recovery and allows increase in peak work rate and endurance, individual training schedule, prevention physical overload and sport injuries. Exhaled breath condensate (EBC) is a complex mixture under influence both of respiratory pathology and environmental factors. Key metabolites concentration in EBC including lactate is much lower than that in the blood. This provides a need in novel diagnostic methods including biosensors with high stability, sensitivity and selectivity. Recently, biosensors are the leading methods in this field so as substrate specificity of lactate dehydrogenase underlying biosensor technique allows direct measurement in a biological sample without previous handling. Biosensors based on ferric ferrocyanide as an electrocatalizator have been developed in the M.V.Lomonosov Moscow State University; they have maximal selectivity for lactate and other markers exceeding that of platina - based biosensors used worldwide.