A technique is examined for measuring the mass of carbon of both ambulatory and non-ambulatory subjects. The method is based upon the detection of the 4.43 MeV gamma rays emitted from carbon nuclei when the body is irradiated with fast neutrons. The supine subject is irradiated laterally by a horizontal collimated beam of 14 MeV neutrons and the emitted gamma rays are counted by a shielded NaI(Tl) detector placed underneath the subject. The method has been calibrated for all sizes of subjects from 30 to 90 kg and there appears to be no significant interference from the other bulk elements of the body. Body carbon has been measured in six normal volunteers; body nitrogen was also determined by measuring the 13N induced in each subject in a second irradiation. Body fat was estimated from the measured carbon and nitrogen, and the values compared with those derived from skinfold thicknesses. The technique is proposed as a method of estimating long-term energy expenditure.