The level of natural killer (NK) cytotoxic activity was measured during co-cultivation of human lymphocytes and target cells (K- 562) in microgravity. Flight experiments were carried out using special instrumentation, the “Fibroblast-1” cassettes, in the frame of Russian scientific program during six ISS missions. Lymphocyte suspensions from human venous blood were used in experiments during short-term flights on six ISS missions (7 –12). Russian space crew members performed the experiments after Soyuz docking. The first step was mixing lymphocytes and 3H-labeled K-562 cells and their incubation at 37°C during 24 hs; the second step was filtration of the cell suspension. The frozen medium and filters were analyzed for the cytokine level and cytotoxic activity after landing. It was found that lympho- cytes with different basal levels of cytotoxic activity kept the ability of recognizing and lysing malignant cells. In micrograv- ity, cytotoxity increased to 160% of the basal levels. Donor indi- vidual features modulated the magnitude of the increase. The measurement of interleukin levels (TNF-α, IL-1, IL-2) in medi- um showed that synthesis of TNF-α increased during cell co-cul- tivation in microgravity. The level of IL-2 was very low in flight and ground control samples. The production of IL-1 by lympho- cytes decreased after in-flight incubation. The results indicate that microgravity did not disturb the cytotoxic function of immune cells in vitro during 24 h incubation with specific target cells.