A study has been made of the physicochemical basis of the early local tissue reaction elicited by repeated subcutaneous injections of water-soluble food colourings in the rat. With the exception of Erythrosine BS, the colourings were all absorbed equally rapidly and uniformly, whether from undamaged or from grossly fibrosed subcutaneous tissue. Hence retention at the injection site could not account for the observed differences in local reaction elicited by the colourings investigated. The physicochemical properties found to play a significant part in determining the early local reaction were surface activity, lipid solubility and protein-binding ability, particularly when the latter was associated with alteration of protein structure. No evidence could be found of aggregation of the colouring or of its ability to interact with calf-thymus DNA in vitro. The course of erythrocyte haemolysis and disintegration of cholesterol-lecithin mixed micelles under the influence of increasing concentrations of the sodium salts of Patent Blue V and Blue VRS appeared identical with the corresponding effects brought about by the surfactants sodium lauryl sulphate and triton X-100. Other triphenylmethane colourings possessing considerable surface activity were Light Green SF Yellowish, Fast Green FCF and Brilliant Blue FCF. Most xanthene colourings tested possessed high solubility in tricaprylin and in cholesterol-lecithin mixed micelles. While not displaying surface activity, these colourings affected the integrity of lipid membranes at very low concentrations, in a manner closely similar to that of the amphipathic compounds saponin and lysolecithin. The capacity to bind to protein and, in so doing, to alter protein structure resided in the colourings Light Green SF Yellowish, Fast Green FCF, Brilliant Blue FCF, Erythrosine BS and Indigo Carmine. None of the physicochemical properties described was present to any appreciable extent in the calcium salts of Patent Blue V or Blue VRS, in the xanthene Violamine R or in any azo colouring studied. The physical properties of a colouring were found to correlate closely with type of local reaction elicited by it. In the presence of a marked degree of surface activity, or of lipid solubility, extensive tissue necrosis resulted initially and determined the development of a type IV reaction. Binding with, and alteration of, protein modified the tissue changes by provoking a macrophage response and so leading to a type III reaction. Erythrosine BS was exceptional in that protein binding considerably diminished its lipophilic property, with consequent reduction in the degree of local damage. Colourings possessing none of the physical attributes described produced a mild effect (type I reaction) while a self-limiting reaction (type II) resulted if these properties were present only to a minimal degree. Our investigations lead to the conclusion that the type of the early local tissue reaction produced and, ultimately, the formation of sarcomas by some of the food colourings studied can be fully accounted for on the basis of the physicochemical characteristics of these compounds.