We examined the effect of rotation-induced stress on (1) growth of lymphosarcoma tumors; (2) interleukin-2 (IL-2) production; (3) T cell subset distribution; and (4) cytotoxic T cell (CTL) function. In addition, we examined the levels of corticosterone and beta-endorphin as possible mediators of stress-induced immune alterations. Rotation stress induced progressive lymphosarcoma growth, while unstressed mice showed tumor regression after 2 weeks of growth. IL-2 production and CTL activity in stressed animals were significantly lower than controls during the first 2 weeks after initiation of stress. Spleen lymphocytes from stressed and control mice bearing the L3T4 antigen (helper/inducer T cell marker) remained unchanged, while in peripheral blood such cells decreased in stressed but not control animals. This latter pattern was also observed in Lyt 2 positive (suppressor/cytotoxic) cells of both spleen and peripheral blood. Corticosterone levels were elevated for an extended period following initiation of stress, while beta-endorphin levels remained similar to those of the controls. Although these data do not directly establish a causal link between immunoinhibition and tumor growth, they clearly demonstrate that stress inhibits a number of cell-mediated immune functions that may be relevant in this regard.