Upon heat shock, Drosophila Kc cells still contain normal cellular messenger RNAs in the cytoplasm. The distribution of these 25 degrees C mRNAs between polysomes and the postpolysomal fraction of heat-shocked cells appears unaltered as compared with control cells. The translatability of these normal cellular messages isolated from heat-shocked and non--heat-shocked Kc cells is unaltered when analyzed by in vitro translation in the rabbit reticulocyte lysate. In contrast, homologous cell-free translation systems obtained from Kc cells effectively discriminate between the in vitro translation of normal cellular messages and heat-shock--specific mRNAs. In particular, a cell-free system from heat-shocked Drosophila Kc cells almost completely shuts down the translation of 25 degrees C messenger RNA species, whereas the translatability of heat-shock--specific messenger RNA appears to be unaffected.