Three lab. tests were used for studying a sewage susceptibility to aerobic biodegradn. The O-2 intake rate was detd. by measuring the O conc. in the sewage each 30 s for 7 min. The fractionation of org. matter was carried out by filtration (0.45 mu m filter) to separate suspension and detn. of BOD and COD in the filtrate and filter cake. The respirometric test was carried out for 28 days to det. the O-2 amount consumed for oxidn. of org. matter in the sewage. The detn. of COD in sewage fractions was fund the most perspective method for assessment of sewage quality in respect to designing the systems for biolog. treatment of sewage.