Total body electrical conductivity (TOBEC) has been introduced as a rapid, safe, and noninvasive method suitable for the estimation of fat-free mass. The instrument (EMME or TOBEC) operates on the principle that organisms placed in an electromagnetic field perturb the field to a degree that depends on the amount and volume of distribution of electrolytes present. A study was designed to measure body composition in infants by the TOBEC method and to compare the results with those obtained using the isotope dilution technique. Sixteen infants (age range, 2 days to 9.7 months; weight range, 2 to 8.7 kg) were enrolled. Total body water (TBW) was determined by the isotope dilution technique using H218O. There was a good correlation between the natural logarithm of the TOBEC number and TBW, with a linear correlation coefficient of 0.949. The fat-free body mass of the infants was calculated by TBW (fat-free body mass = /0.082) and by the TOBEC method using the standard previously derived from mature rabbits. TBW measurements by H218O dilution appeared to overestimate fat-free mass which was greater than TBW in five of the 16 infants. Measured by the TOBEC method, fat-free mass ranged from 51 to 91% of total body weight. The TOBEC method is highly suitable for use with human infants and appears to determine body composition as accurately as other available methods.