The humoral immunosystem is impaired in those cases of catabolic metabolism in which quantitative and qualitative insufficient alimentation is expected after operation. 2. We showed that after cardiac surgery with extracorporal circulation as well as after other operations a reasonably balanced parenteral nutrition composed of carbohydrates and amino acids could considerably inhibit the changes in the humoral immunity, which is a valuable contribution to the prevention of infection. 3. In our experiments on transformation of lymphocytes after mitogenic stimulation, the depressive effects of operation on the cellular immunity were found to be etiologically more complex than was previously assumed in publications.