Abstract: A method of screening a test substance for potential teratogenicity without employing vertebrate animals. In a representative embodiment, Drosophila primary embryonic cells, containing a hybrid game including a coding sequence for a detectable expression product under the control of a promoter sequence selected from among the 23 and 22 kDa Drosophila heat shock protein promoter sequences, are exposed to the test substance. The cells are thereafter assayed for the detectable expression product, the presence of the expression product in the cells providing an indication of the potential teratogenicity of the test substance. The primary embryonic cells containing the hybrid gene are conveniently provided by transformed flies.