The response of intact Drosophila-fruit-flies to known teratogens was evaluated. Seventeen chemicals were tested. Fly larvae were treated over their entire morphogenic cycle by adding each test chemical to the culture medium. Eggs were deposited by untreated females on the surface of the medium and hatched, and the emerging larvae immediately began to feed. After several days these larvae metamorphosed into adult flies. The incidence of abnormalities among exposed flies was compared with the incidence in control flies. An increased incidence of 14 kinds of abnormalities occurred in flies that were treated with the test chemicals. Although there was a variety of responses, each chemical tested elicited a unique and reproducible pattern of abnormalities that appeared generally to be dose related. For example, vincristine-sulfate (2068782), 5- fluorouracil (51218), and methotrexate (59052) induced eye defects. Vincristine-sulfate caused a bulging or rippling of eye tissue. 5- Fluorouracil reduced the size of the eye. Methotrexate disrupted the anterior half of the eye surface. Of the tested chemicals, only colchicine (64868), coumarin (91645), and ethanol (64175) produced weak responses. The only positive response to colchicine was a significant reduction in bristle size in three of nine flies. Ethanol increased slightly the incidence of abdominal irregularities. The authors suggest that since Drosophila appears to be resistant to ethanol, ethanol may be a reasonable choice as an aid in solubilizing test chemicals for incorporation into the medium. Drosophila may have the potential to be a valuable teratogen screen. Additional studies, especially with nonteratogenic substances, are necessary.