An innovative, safe, and rapid method suitable for the determination of lean body mass in infants is described. The machine (EMME) operates on the principle that a biologic specimen in an electromagnetic field perturbs the field proportional to its conductivity which is dependent on the amount of electrolyte present. The physical characteristics of the EMME signal are described. Experiments done with phantom infants composed of electrolyte solution and corn oil showed a very linear relationship between fat-free volume and the natural log of the EMME signal (r = 0.97). A similar relationship was noted for ground beef (r = 0.91) and live rabbits (r = 0.99). The EMME instrument appears to hold promise as a rapid noninvasive means of determining lean body mass in infants.