Exposure to microgravity may produce changes in the performance of the immunological system at the cellular level and in the major physiological systems of the body. Weightlessness suppresses the lymphocytic functions involved in the immunity process, such as cell locomotion and expression of antigen. The present study was designed to investigate whether the proliferation and viability of lymphocytes are reduced by exposure to rotation in a three-dimensional (3-D) clinostat, which is used to simulate the microgravity of cells. The results indicate a nonsignificant decrease in the proliferation and cellular viability to the mitogen stimulation in 24 h of simulated weightlessness (P = 0.146). There was, however, a very significant (P = 0.012) decrease in proliferation and viability after 48 h of rotation in the 3-D clinostat. A comparison between 24 and 48 h of clinorotation indicates a difference between the results (P = 0.003). The present study indicates that the immunological depression associated with the spaceflight is not just related to the psychological and physiological stresses that the astronauts are subjected to, but it also seems to be caused by microgravity per se that affects the proliferation and cellular viability.