1. The fat-free mass (FFM) of ten male and ten female subjects was calculated from measurement of (i) height and weight, (ii) skinfold thickness, (iii) body density, (iv and v) total body potassium by two different methods. All the subjects were apparently healthy and in the medium range of body build and they were mainly young adults.
2. The mean FFM of the ten male subjects was similar as calculated by all methods, although comparisons between pairs of methods sometimes produced discrepancies of up to 2 kg.
3. Agreement was less good for the females but, with the exception of one of the potassium methods, still reasonable.
4. The potassium content of the FFM as mEq K/kg of FFM, was compared with the direct analysis of Forbes, Gallup & Hursh (1961) on four male cadavers. The agreement was close for the men but the K content of the FFM in women was lower than that for men. This difference is discussed.