1. The humoral immunosystem is impaired in those cases of catabolic metabolism in which quantitative and qualitative insufficient alimentation is expected after operation. 2. After cholecystectomies as well as after cardiac surgery with extracorporal circulation a reasonably balanced parenteral nutrition composed of carbohydrates and amino acids would 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 of the cellular immunity were found to be etiologically more complex than was previously assumed in publications.