Eight healthy young men participated in a metabolic balance study designed to assess the protein quality of soy protein concentrate (STAPRO-3200). Subjects received varying intakes of the test protein [0.35, 0.45, 0.55 and 0.65 g protein (N X 6.25) per kilogram body weight per day] during 10-day experimental periods. Each was preceded by a 1-day protein-free diet. A final diet period with a 0.65 g milk protein intake was included for comparative purposes. During this period and that with the 0.65 g test intake of soy concentrate the stable isotopes 70Zn and 58Fe were added to the diet for 2 days to obtain initial data on the absorption of zinc and iron when soy concentrate was the sole source of protein intake. Mean (+/- SEM) intake of soy concentrate determined to be sufficient for N balance (including allowance of 5 mg N/kg per day for integumental and miscellaneous losses) was 95 (+/- 7) mg N/kg per day. Absorption of the extrinsic doses of labeled iron and zinc did not differ between the soy protein- and milk-based diets. Comparison of these data was made with findings obtained previously with other protein sources evaluated in the same way. The conclusion is that the capacity of the soy concentration to support short-term nitrogen equilibrium in adult protein nutrition is the same as that for good quality animal protein sources. Thus, well-processed soy concentrates can make a nutritionally significant contribution to meeting adult human protein needs.