OBJECTIVE To determine if induction of heat shock protein 70 (HSP 70), a stress protein that plays a cytoprotective role and inhibits cell death in response to various stimuli, will protect thymocytes and T-cell clones from radiation-induced apoptosis, and to define the mechanism of such protection. METHODS Thymocytes from BALB/c mice or T-lymphocyte clones were incubated at 43 degrees C for 1 hour to induce HSP 70, then irradiated. Control cells were irradiated but not heated. Fragmentation of DNA was quantitated, and p53, bax, and bcl-2 expression was analyzed at various times by the Western blot method. RESULTS Only heated cells expressed HSP 70. The induction of HSP 70 increased basal apoptosis but significantly decreased radiation-induced apoptosis. Furthermore, introduction of an HSP 70 antisense oligomer prior to heating reversed the protective effect of HSP 70. Induction of HSP 70 in T-cell clones with sodium arsenite had a similar protective effect against radiation-induced apoptosis. Irradiation induced p53 and markedly up-regulated bax. The expression of p53 peaked at 4 hours and preceded maximal bax induction. Induction of HSP 70 prior to irradiation suppressed p53 and significantly decreased bax levels. Levels of bcl-2 were unaffected. CONCLUSIONS Our data show that HSP 70 induction protects thymocytes from radiation-induced apoptosis by down-regulating p53 and bax expression. The induction of HSP 70 may represent a novel mechanism by which the immunosuppressive effects and the associated infectious complications of radiation therapy can be minimized.