Chemical compounds known to be teratogenic to frog embryos were tested singly and in binary mixtures and Weibull functions were used to model their concentration-response relationships. A separate Weibull function with an additive concentration variable modeled the mixtures using only single chemical test data. Seven chemicals in five binary mixtures at 3/1, 1/1, and 13 mixture proportions each were tested. The ratios of measured and calculated model parameters and median concentrations were estimated for comparing the results. The ratios ranged from 0.72 and 1.44 with an average at 0.99, an agreement that was qualitatively similar to modeling mixtures of narcotic chemicals which are known to produce additive concentration effects. The model reliability in determining mixture response classes and prediction of effects based on single chemical data was quantified. The model is useful for environmental risk assessment of chemical mixtures in hazardous waste sites and for the design of mixture experiments.