First studies of the labile organic matter (OM) (carbohydrates, lipids, proteins) in different parts of Lake Onega which differ in the trophic status, discharge from rivers and human impact were carried out. These compounds can serve as indicators of the ecological status of the lake. Carbohydrates prevail among labile OM. Their content in the oligotrophic parts of the lake was on average 2.2 mg/l, and twice higher in the anthropogenically eutrophic bays. The concentrations of lipids and proteins in the lake were on average 0.35 and 0.32 mg/l, respectively, and their content was a little higher in the bays than in the lake center, due to the higher trophic levels in bays compared with the pelagic zone. The content of lipids in the lake was the highest in spring.