Of five glycolytic enzymes tested, only the serum activity of phosphohexose isomerase (PHI) was elevated in the vast majority of 140 patients with gastrointestinal, kidney or mammary carcinomas. However, in the tumor tissues all enzymes were increased to about the same extent. Thus, the measured increase in the serum activity of PHI is not due to a specific overproduction in the malignant cells. A general rise in the glycolytic enzyme activities in the malignant tissue could be detected already in early stages without metastases and in well-differentiated tumors.