Cellular stress eliminates irreversibly damaged cells by initiating the intrinsic death pathway. Cell stress is sensed by pro- and antiapoptotic members of the Bcl-2 protein family, which regulate the release of apoptogenic factors, such as cytochrome c, from mitochondria. Exposure of cells to hyperthermia results in the activation of the proapoptotic Bcl-2 family protein Bax, which plays an essential role in cytochrome c release. Heat directly affects Bax activity in vitro; however, antiapoptotic Bcl-2 family proteins, such as Bcl-xL, can suppress this activation, suggesting that a second heat-sensitive step must be breached before apoptosis ensues in cells exposed to hyperthermia. Here we show that heat shock causes the loss of Mcl-1 protein. Depletion of Noxa by short hairpin RNA protected cells from hyperthermia by preventing Mcl-1 degradation. Heat shock caused the dissociation of Noxa from Mcl-1, which allowed binding of the BH3-containing ubiquitin ligase Mule followed by Mcl-1 ubiquitination and degradation. Overexpression of Hsp70, which prevents heat-induced Bax activation, stabilized Mcl-1 protein levels in heat-shocked cells. This resulted from reduced Mule binding and ubiquitination as well as enhanced Mcl-1 expression compared with cells without Hsp70. Our results demonstrate that loss of Mcl-1 is a critical heat-sensitive step leading to Bax activation that is controlled by Hsp70.