With a view to the possible use of human tumor cells for diagnostic and therapeutic tests (e.g. determination of hormone receptors, sensitivity to cytotoxic drugs, cytotoxicity tests), an attempt has been made to isolate and cultivate pure human tumor cell populations. 49 malignant effusions from 42 patients formed the starting material. The tumor cell isolation was carried out in three steps: 1. elimination of erythrocytes by density-gradient centrifugation or osmotic lysis with triple distilled water; 2. concentration of tumor cells by lg-sedimentation; 3. elimination of adherent cells by short term cultivation in plastic tissue culture flasks. This procedure yielded 95-98% tumor cells. From these suspensions of purified tumor cells, primary cultures were successfully established by supplementing the tissue culture media with autologous effusion fluid.