2D-electrophoresis performed as described by O'-Farrel has revealed clear-cut differences in the pattern of proteins synthesized in the cells of wild-type flies and in the cells of ts-lethal. The cells of the mutant studied after heat-shock exhibit not only the lack of HSP83 and HSP35 but also the absence of a few of the heat-shock proteins belonging to the HSP70 group. Moreover, in the cells of the mutant an intensive synthesis of a protein with mol. weight 72 KDa was observed after heat-shock. This protein belongs to highly abundant heat-shock cognate proteins (HSCP) which are usually not induced by temperature elevation.