The nutritional quality of soy protein isolate (SPI) was evaluated in young men by the nitrogen balance method using fish protein as a standard. Twenty-one male university students were given SPI (Supro 620, Ralston Purina Co., St. Louis, Mo., U.S.A.), fish protein (cod fish) or a 50:50 mixture of the two proteins as the sole source of protein. SPI and fish were prepared as paste products (kamaboko). Four experimental periods were used in which each subject received 0.35, 0.45, 0.55 and 0.65 g protein/kg/day, respectively, given in random order. Each period consisted of one day on protein-free diet and ten days on the experimental diet, with an interval of three days on a free-choice (ad libitum) diet between periods. Energy intake was constant for each individual to maintain their body weight (44.6 +/- 2.4 kcal/kg/day). The linear regression equations obtained between nitrogen intake (X: mg N/kg/day) and the apparent nitrogen balance (Y: mg N/kg/day) were as follows: SPI, Y = 0.298X-35.2; fish protein, Y = 0.365X-31.8; mixed protein, Y = 0.423X-38.3. The nitrogen requirement for maintenance of nitrogen equilibrium determined from the regression equation was 118.1 +/- 15.4 mg N/kg/day for SPI, 87.1 +/- 17.2 mg N/kg/day for fish protein and 90.5 +/- 17.1 mg N/kg/day for mixed protein. The NPUs calculated at the respective maintenance nitrogen intakes were 38.9, 52.8 and 50.8 for SPI, fish protein and mixed protein, respectively. There was no significant difference between the nutritive values of mixed protein and fish protein. The nutritive value of SPI relative to fish protein was estimated as 82%, 74% and 74% by the slope-ratio method, and values for the nitrogen requirement and NPU, respectively.