Eukaryotic cells respond to elevated temperatures by rapidly activating the expression of heat shock genes. Central to this activation is heat shock-inducible binding of the transcriptional activator, termed heat shock factor (HSF), to common regulatory elements, which are located upstream of all heat shock genes. The DNA binding activity of the inactive form of Drosophila HSF was induced in vitro by treatment with polyclonal antibodies to the purified, in vivo-activated factor. This finding, together with observations that high temperature and low pH activate HSF binding in vitro, suggests that the inactive form of HSF can directly recognize and transduce the heat shock signal without undergoing a covalent modification of protein structure.