A study was done to determine the influence of cancer-bearing patients’ plasma (CPP) on lymphocytic cortisol metabolism in comparison with the plasma of healthy donors. Known concentrations of human lymphocytes were incubated with cortisol in media containing 50% phosphate-buffered saline (PBS) and 50% of one of the following additions: A) homologic plasma (HP), b) heterologic plasma (HetrP), c) HetrP obtained from patients who had diseases other than cancer and who were over the age of 70 years, <Q CPP, e) boiled HP, f) boiled CPP, g) mixed plasma comprising 50% HP and 50% CPP, and h) PBS. All the plasmas had the capacity to enhance the lymphocytic cortisol metabolism rate when compared with the ability of PBS. There was no difference between the conversion rates obtained with the use of HP of healthy donors and those obtained with the use of HetrP of healthy donors, nor was there a difference between the rates obtained with the use of plasma of elderly patients and HP. CPP, however, led to significant reduction In activity. Mixing CPP with HP revealed that the reduced effect of the CPP was due to the lack of a factor or factors rather than the presence of inhibitory factors. Boiling of the HP and the CPP before incubation showed the presence of a temperature-labile factor in the HP and its absence in the CPP. This finding of a difference in the presence of a lymphocytic cortisol metabolism-enhancing factor between the plasma of cancer patients and the plasma of healthy donors may provide the basis for a new diagnostic aid for the early detection of cancer.