The fundamental biologic determination of corporeal specific gravity, essentially a relationship between weight and unit volume, has been neglected in the modern classification of healthy persons. Stern1and Spivak2emphasized the value of the measurement of corporeal density, but their experimental data are not conclusive. Of especial interest is the relationship between specific gravity and the fat content of the body. The presence of an indeterminate amount of excess adipose tissue renders difficult any precise computation, for example, of metabolic rate or dosage of drugs in terms of total body weight. The important consideration should be the weight of the lean body representing the active mass of protoplasm. In this paper the data support the concept that the comparatively low specific gravity of fat makes the measurement of the specific gravity of the body mass valid for the estimation of fat content. The comprehensive, statistical analysis of Boyd,