subsequent cell growth was decreased at concentrations of 13-cis-RA greater than 1 X 10(-5) M. After a 40-hr treatment period, labeling indices in retinoid-treated cells were significantly lower than control values (25% compared with 40%). Retinoic acid also caused a significant, concentration-dependent decrease in 3H-thymidine incorporation. The inhibitory effect of 13-cis-RA on proliferation of oral-nasal mesenchymal cells appears to be related to the production of craniofacial malformations.