Crosslinkage of the B cell antigen receptor by anti-mu beads or SAC results in the selective induction of hsp70. We have observed that activated cells, having enhanced expression of hsp70, survive lethal stimuli much better than their unactivated counterparts. These results are in accordance with the proposal that hsp70 is essential for cells to survive lethal environmental stresses. Moreover, the activation event itself primes B cells thereby enabling them to increase the expression of both hsp70 mRNA and protein. This is the first demonstration that triggering of B cells via crosslinkage of sIg is accompanied by the induction of thermotolerance without the need for a prior sublethal heat treatment.