Exposure of prokaryotic and eukaryotic cells to heat shock (hyperthermia) or to a number of diverse environmental stresses such as teratogens, anoxia, and inhibitors of oxidative phosphorylation results in the enhanced synthesis of a number of proteins which have been previously referred to as heat shock proteins (hsps). More recently, in view of the diverse types of agents that can induce these proteins, they have also been referred to as stress proteins. This phenomenon is one of the most basic regulatory mechanisms in living organisms. Exposure of Drosophila embryos, larvae, or pupae to these types of stresses also results in a variety of developmental abnormalities in the ensuing adult. Although the function(s) of these heat shock proteins has yet to be determined, they are widely thought to play an important role in cell survival and protection following some types of environmental stress. In our laboratory, we have developed an in vitro assay for detecting agents that act as teratogens, utilizing Drosophila embryonic cultures. Drosophila embryonic cells differentiate in vitro to a number of functional cell types including myotubes and ganglia. A number of drugs that have been shown to act as teratogens in mammals have also been found to inhibit muscle and/or neuron differentiation in Drosophila embryonic cultures. We have examined, by two-dimensional gel electrophoresis, the effects of such teratogens on protein synthesis in Drosophila embryonic cells. Inhibition of muscle and/or neuron differentiation correlates well with the induction of two proteins of about 20 kilodaltons. These are identical to two of the heat shock proteins (hsp 23, 22) as shown by electrophoretic mobilities and peptide mapping by partial proteolysis. Heat shock and other treatments such as exposure to some of the metal ions and ether induces the entire set of seven major heat shock proteins in the Drosophila embryonic cells. Dose-response studies of several teratogens show a correlation between the degree of inhibition of differentiation and the level of induction of hsps. Since heat shock proteins have been suggested as possibly serving a protecting role, our present studies are aimed at identifying the role of hsps in teratogenesis and investigating the differential regulation of heat shock genes in response to different external stimuli.