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The fat content of living subjects was determined by a model of human body composition based upon total-body 40K counting but without the assumption of a constant potassium concentration in the lean body mass (adipose-free mass). Calibration of the human counter had been accom­plished by the administration of 42K and other gammy-ray emitters to hu­mans of various sizes and ages with the concurrent comparison of their counting rates per retained unit of radioactivity against those of radioactive standards counted in a series of polyethylene phantoms of varying thickness.Those phantoms representing obese individuals had thick layers of non­radioactive absorber surrounding the radioactivity. The body composition model includes data for individuals of both sexes and for ages from newborn to adult. Results are compared with those obtained by other investigators, most importantly from studies of cadaver composition and also from studies of living persons by anthropometry, densitometry, etc. A conflict among authors concerning the fat content of normal and obese subjects is reviewed. Application of the model to investigation of patients receiving medical or surgical treatment is described for morbid obesity, kidney disease, liver disease and metabolic disorders.