Experiments in space have shown that T lymphocyte function is altered in more than 50% of space crew members. There is strong evidence that such effect is due to stress rather than to weightlessness per se. However the health of astronauts was never threatened so far. Experiments in-vitro with cultures of human peripheral blood lymphocytes (not from astronauts) have shown that T cell function is dramatically reduced. Recent work with the random positioning machine, a new instrument to simulate conditions similar to microgravity, indicate that there are direct gravitational effects on the genetic expression of interleukin-2 and of its receptor in T lymphocytes.