A body composition study of 31 healthy subjects covering a wide range of age (23.5-72.0 years) and weight (44.5-104.2 kg) has been undertaken. Subjects were assessed by in vivo neutron activation and tritiated water analysis and values of total body nitrogen, hydrogen and fat obtained by utilization of a five-compartment model of body composition comprising protein, water, fat, minerals and glycogen. The protein (as 6.25 x nitrogen) and water compartments were measured but the smaller compartments of minerals and glycogen were calculated as fixed fractions of the fat-free mass estimated from the water space. Fat was calculated as the body mass less the sum of the four other compartments. Mean values (+/- SEM), expressed as a percentage of body mass, for nitrogen, hydrogen and fat were 2.56 (+/- 0.07)%, 10.07 (+/- 0.04)%, and 21.9 (+/- 1.7)% respectively for men and 2.14 (+/- 0.07)%, 10.40 (+/- 0.04)%, and 35.5 (+/- 1.7)% respectively for women. The accuracy of the nitrogen measurements was evaluated by comparison with calculated values from two prediction equations; correlation coefficients, the mean bias (estimated from the mean differences between the measured and predicted nitrogen), the confidence interval for the bias, and limits of agreement were calculated. The correlation coefficients were high (r > 0.93) and the mean bias indicative of agreement. The ratio of nitrogen to the fat-free mass (derived from the body composition model) was also calculated and mean values (+/- SEM) of 32.7 (+/- 0.4) and 33.1 (+/- 0.4) g/kg for men and women, respectively were obtained. The hydration of the fat-free mass was determined to be 0.725 (+/- 0.002) and 0.722 (+/- 0.002) kg/kg for men and women respectively. The accuracy of the body fat estimate was evaluated by comparison with skinfold-thickness-derived values and computation from tritiated water space. The ratio of the body composition model to skinfold-thickness-derived fat was significantly (P < 0.005) greater than unity. The mean bias between the body composition model and tritiated-water-derived fat was -0.6 percentage points of fat (95% confidence interval from -0.3 to -0.9 percentage points of fat). Finally a prediction equation (r2 = 0.908, SEE = 108 g) for body nitrogen in healthy subjects based on weight, age and sex was calculated.