The purpose of this study was to compare the effects of hypergravity exposure (2g) with those of exposure to space flight in the Cosmos 2044 flight. To do so, rats were centrifuged continuously for 14 days. Two different experiments were carried out on tissue obtained from the centrifuged rats. In the first experiment, rat bone marrow cells were examined for their response to recombinant murine colony stimulating factor-granulocyte/monocyte (GM-CSF). In the second experiment, rat spleen and bone marrow cells were stained in with a variety of antibodies directed against cell surface antigenic markers. These cells were preserved and analyzed on a flow cytometer. The results of the studies indicated that bone marrow cells from centrifuged rats showed no significant change in response to GM-CSF as compared to bone marrow cells from control rats. Spleen cells from flown rats showed some statistically significant changes in leukocytes subset distribution, but no differences that appeared to be of biological significance. These results indicate that hypergravity did not greatly affect the same immunological parameters affected by space flight in the Cosmos 2044 mission.