Ultracentrifugal analysis was used to study aggregation of 7S globulin induced by ultrasonic treatment, of water extracts from defatted soybean flakes. In pH 7.6, 0.5 ionic strength buffer, aggregated 7S proteins sedimented as 40–50S species and represented 25–40% of the total proteins. Aggregates also existed at low ionic strength, but dialysis at 0.5 ionic strength caused additional aggregation of sonically modified 7S proteins. After exposure to 0.5 ionic strength, aggregates were stable to subsequent changes in ionic strength with one exception; some reversal of aggregation occurred when samples were dialyzed against water. Aggregates exhibited highest stability at pH 6.5–6.8 and were stable at room temperature for 9 hr or more. Sonic-induced aggregation of 7S proteins resembles the phenomenon observed on heating water extracts at 80°C.