When lymphocytes are isolated from blood using the Böyum method a sometimes severe contamination with non-lymphoid cells (NLC) is observed in cancer patients, but also in patients with non-malignant diseases. These non-lymphoid cells consist mainly of monocytes and granulocytes. A large monocytic contamination is found in cancer patients, while the percentages of monocytes in a patient group with benign diseases and in a normal donor group were equally lower than in the group of cancer patients. The granulocytic contamination covered a large range in both patient groups and the mean percentages did not differ. When lymphocyte studies are to be performed in vitro, allowances have to be made for these varying percentages of non-lymphoid cells. In addition a high percentage of non-lymphoid cells makes the presence of a malignant tumor more likely.