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Body potassium was measured in 135 young male and 161 young female subjects (mean age 20 years) in good health by a whole body counter of good geometry. Because of the unusual design of this counter, the conditions of whole body counting procedures (calibration, precision, accuracy) are given in detail. Determinations of lean body mass were made by two independent methods: 1. tritiated water dilution and 2. anthropometric method: equations based on tritiated water dilution and on anthropometric measurements give remarkably concordant values for lean body mass. Moreover, their correlation with total body potassium is very good. This correlation enabled the ratio of potassium to lean body mass (mEq/Kg) to be determined in the population under study; the mean ratio was found to be significantly lower than the standard value and different in the two sexes (men: 62.7 mEq/Kg; women: 56.8 mEq/Kg). Because there is a wide margin of variation around the mean values, the accuracy of the indirect methods used for computing lean body mass is discussed. However, the use of regression equations makes it possible to predict total body water and total body potassium with good precision from certain anthropometric measurements. However, from a clinical viewpoint, there appears to be no reference index for body potassium of unquestionable value.