Heat shock protein (HSP) 70 can be released from tumor cells and stimulate a potent antitumor immune response. However, HSP70 does not contain a consensus secretory signal and thus cannot traverse the plasma membrane by conventional mechanisms. We have observed HSP70 release from intact human prostate carcinoma cell lines (PC-3 and LNCaP) by a mechanism independent of de novo HSP70 synthesis or cell death. This pathway is similar to one used by the leaderless protein IL-1β. Our studies show that HSP70 release involves transit though an endolysosomal compartment and is inhibited by lysosomotropic compounds. In addition, the rate of HSP70 secretion correlates well with the appearance of the lysosomal marker LAMP1 on the cell surface, further suggesting the role for endolysosomes. The entry of HSP70 into this secretory compartment appears to involve the ABC family transporter proteins and ABC transporter inhibitor glibenclamide antagonizes secretion. Although the cell signals involved in triggering stress induced HSP70 release though this lysosomal pathway are largely unknown, our experiments suggest a regulatory role for extracellular ATP. These mechanisms appear to be shared by IL-1β secretion. Following release, we observed the binding of extracellular HSP70 to the cell surface of the prostate carcinoma cells. These findings suggest that secreted HSP70 can take part in paracrine or autocrine interactions with adjacent cell surfaces. Our experiments therefore suggest a mechanism for HSP70 secretion and binding to the surface of other cells that may be involved in recognition of the tumor cells by the immune system.