Successful use of selective herbicides is dependent upon differences in susceptibility between the crop and the weeds. Often the cause and magnitude of these differences is not completely known. Intelligent use of a chemical requires some knowledge of the margin of safety between the weed and the crop. Beneficial modifications of existing chemicals or methods of application could be most efficiently accomplished if the cause of the differences in susceptibility to the chemical were known. The alkanolamine salts of 4,6–dinitro o secondary butylphenol (hereinafter referred to as DNBP) have shown variability in their toxicity to various plant species. This investigation is concerned with the degree and possible causes of this variability.