The electrophoretic mobility of soluble collagen of rat tail tendon, adsorbed on oil droplets, has been measured in a range of salt solutions between pH 1 and pH 12, maintained at constant ionic strength. Specific effects of the lower valent buffer ions used appear small and the corresponding mobilities lie on a smooth curve. These mobilities parallel the hydrogen ion binding expected from the amino acid composition, with a long isoelectric zone in the range pH 7.7 ± 0.8, in agreement with reported isoelectric points of native collagen. Addition of the divalent cations: Pb ++, Cu ++, Ca ++, Mg ++, or ofthe anions: citrate, phosphate, pyrophosphate, oxalate, at constant ionic strength shifts the mobility, in order of decreasing effect, to more positive or negative values respectively, with consequent shortening and raising or lowering of the isoelectric zone. These specific effects are attributed to binding of these ions by the protein, and the quantitative interpretation is discussed.