In order to clarify the effect of the unilateral pneumonectomy on the compensatory growth of the residual lung, liver and kidney, and serum electrophoresis pattern, right lung pneumonectomy was performed on rabbits under general anesthesia with pentobarbital sodium. On the fifth day after the surgery, the weight of the residual lung, liver and bilateral kidneys was measured and organ weight-body weight ratio was calculated. And in an attempt to know whether the cells in the liver and the kidney were proliferated by unilateral pneumonectomy, DNA content was determined. The quantity(g/100 ml) of serum protein was determined also and serum electrophoresis was performed on cellulose acetate membrane. The results obtained are summarized as following. The weight of the residual lung and lung weight-body weight ratio was significantly increased respectively. The weight of the liver and organ weight-body weight ratio were not changed but the DNA content of the liver and kidney tissue increased significantly, illustrating that unilateral pneumonectomy caused cellular hyperplasia in the liver and in the kidney as well as in the residual lung. The quantity(g/100 ml) of serum protein was significantly increased and in the analysis of the electrophoregram, there was significant difference between the normal and pneumonectomy group. Taken together, these results indicate that unilateral pneumonectomy caused the compensatory hyperplasia of the liver, the kidney and the residual lung as well as the change of electro-phoretic pattern. And it also suggests that a humoral factor, which proliferates the cells in the residual lung, the liver and the kidney, existed in the pneumonectomized rabbits.