The coagulation of soy protein isolates (SPI) induced by subtilisin Carlsberg was studied. The proteins were digested to fragments of 16 kDa or less in the early stage of the reaction, followed by coagulation. The time-course of the coagulation measured by turbidity was separated into three phases. The turbidity decreased from the initial level observed at time zero to the minimum level (OD1) at time T1 (15-20 min) in the first phase. Then, it increased drastically to reach the maximum (OD2) at time T2 (60-70 min) in the second phase, which was followed by a slight decrease in the third phase. The coagulation was terminated at T2, where 30-35% of the weight of the SPI proteins was in coagula. Proteins in the coagula were degraded slowly in the prolonged incubation, and the protein content in the coagula was finally 15-20% of the weight. The time-course of the turbidity agreed well with that of the weight of the precipitates formed, indicating that the turbidity reflects the progress of the coagulation. The turbidity change (OD1 to OD2) from the start to the end of the coagulation increased proportionally to the SPI concentration (4.9-11 mg/mL), although the time (T1 to T2) needed for the coagulation was independent of the concentration. The growth of the coagula is promoted by increasing the SPI concentration and is rate-limiting in the coagulation.