Objective:To study the role of inducible form of heat shock protein 70(Hsp70) in the host tumor regression of rat tumor model.Methods:We examined the role of Hsp70 in host tumorigenicity and in vitro cellular cytotoxicity using a rat histocytoma.The differential tumor growth and regression kinetics were studied and correlated with the expression of Hsp70,activation of macrophages and natural killer(NK) cells,and circulating or tumor infiltrating immune molecules in the host system.Results:The sub cuteaneous(s.c.) tumor regression was correlated with increased serum cytokines such as IL-12,TNF P,IFNγand Hsp70.Despite of similar increase of Hsp70 in intraperitoneal(i.p.) tumor implanted animals,animals succumb to tumor growth,further,evidently,no immune molecule activation was observed.The viral promoter driven Hsp70 over expression in these tumor cells restrained solid tumor growth,however,failed to inhibit ascites growth.The NK cells from s.c.immunized animals induces cytotoxicity in the presence of anti-tumor antibody,which necessitated CD40-L expression,conversely,NK cells from i.p.immunized animals failed to induce cytotoxicity.The NK cells from s.c.or i.p.implanted animals with Hsp70 positive tumor cells failed to induce such cytotoxicity.The peritoneal macrophages isolated from s.c.tumor implanted animals when co-cultured with parental BC-8 cells lyses tumor cells,nevertheless entail macrophage specific TNFαexpression.On the contrary,Hsp70 expressing BC-8 tumor cells were resistant to peritoneal macrophage induced cytolysis.Conclusions:This study brings out that Hsp70 possibly involved in regulating the host tumor response and cellular cytotoxicity. Objective: To study the role of inducible form of heat shock protein 70 (Hsp70) in the host tumor regression of rat tumor model. Methods: We examined the role of Hsp70 in host tumorigenicity and in vitro cellular cytotoxicity using a rat histocytoma. Differential tumor growth and regression kinetics were studied and correlated with the expression of Hsp70, activation of macrophages and natural killer (NK) cells, and circulating or tumor infiltrating immune molecules in the host system. Results: The sub cute (sc) tumor regression was correlated with increased serum cytokines such as IL-12, TNF P, IFNγ and Hsp70. Despite of similar increase of Hsp70 in intraperitoneal (ip) tumor implanted animals, animals succumb to tumor growth, further, evidently, no immune molecule activation was observed. viral promoter driven Hsp70 over expression in these tumor cells restrained solid tumor growth, however, failed to inhibit ascites growth. The NK cells from scimmunized animals induces cytotoxicity in the presence of anti-tumor antibody, which necessitated CD40-L expression, conversely, NK cells from ipimmunized animals failed to induce cytotoxicity. The NK cells from scor ipimplanted animals with Hsp70 positive tumor cells failed to induce such cytotoxicity.The peritoneal macrophages isolated from sctumor implanted animals when co-cultured with parental BC-8 cells lyses tumor cells, nevertheless entail macrophage specific TNFα expression. On the contrary, Hsp70 expressing BC-8 tumor cells were resistant to peritoneal macrophage induced cytolysis. brings out that Hsp70 possibly involved in regulating the host tumor response and cellular cytotoxicity.