Heat shock proteins appear to be causatively involved in the acquisition of thermotolerance in prokaryotes but not in eukaryotes. Further, the enhanced synthesis of hsps may be necessary for some cellular responses to stress but not others. In prokaryotic cells the development of thermotolerance, as measured by cell survival, is dependent upon protein synthesis. However, in eukaryotes, enhanced hsp synthesis following an inducing stress and prior to a subsequent heat shock is neither necessary nor sufficient for the development of thermotolerance as measured by colony-forming assays. The enhanced expression of hsps may be required for some mammalian cellular stress responses, such as the ability to reform both actin microfilament bundles and nucleolar morphology. These latter two thermotolerant responses have not been correlated with colony-forming ability. Future work should address the relationships between these various physiological responses to stress and determine if hsps function in some repair mode with regard to colony formation responses. Evidence is accumulating that hsps or their cognates may function in growth and differentiation in some manner as yet to be fully explained. Recent studies indicate that genes controlling cell division in E. coli may be linked to those of several stress regulons, and it would not be surprising to find a similar relationship in eukaryotes. At this time, it is important that studies investigating the role of hsps in stress and other cellular responses such as growth and differentiation define the specific gene (including its regulatory sequences) that encodes the protein being investigated, in order to avoid apparently contradictory and confusing reports of hsps expression.