A teratology test system proposed by Chernoff and Kavlock (3,4) utilizes growth and viability for 3 days after birth to prioritize chemicals for standard teratology testing. The present study is an extended observation of the growth and viability of Chernoff and Kavlock's animals from 41 treatments for 250 days to determine if neonatal weight reductions persisted throughout life, if mortality occurred later in life or, if other serious abnormalities developed that could not be anticipated from the neonatal data. One chemical which was positive in the present study would be a false negative in the standard teratology test and the Chernoff/Kavlock Assay (CKA). Congo red did not cause frank malformations or mortality but it specifically altered reproductive function in the offspring. Two chemicals produced a variety of unusual abnormalities that were not apparent in the neonates, indicating that postnatal testing is necessary to determine the full teratogenic potential of some compounds. Mice exposed prenatally to cytosine arabinoside became hydrocephalic and 30% of them lacked lower incisors.