Interactions of nine dosage rates each of chloramben (3-amino-2,5-dichlorobenzoic acid) and diphenamid (N, N-dimethyl-2,2-diphenylacetamide) were studied using Italian ryegrass (Lolium multiflorum Lam.) as a test species. Injury score, estimated percent stand reduction, height, and fresh weight data were taken 1 week after seeding. Techniques for developing linear regression models based on these data and evaluating them graphically in terms of completeness and statistical validity are discussed. Three-dimensional graphs of these models indicate areas of possible herbicide interaction.