Certain reactive dyes of the procion M and remazol group were effective for vital staining of growing bones. These compounds appear to form covalent bonds with the protein matrices and are preserved in situ after decalcification. Dye concentrations in sera of animals receiving one injection were followed. There was a precipitous drop in optical density in the first 24 hours; the remainder of the dye was largely cleared from the serum in 11–21 days. Dye concentrations and staining of bone were correlated. The width of the stained bone appeared to be related to rate of growth and disappearance of dye from the blood. On electrophoresis, the dyes moved with the albumin fraction. Dialysis and electrophoresis experiments favored the conclusion that they form covalent bonds with the protein. Growth of the rabbit mandible at 5–13 weeks, was studied by microscopy in decalcified sections. Using multicolored dyes, the sites of growth were marked in known sequence and sites of resorption were identified by interruption of stained zones. Principles of growth and remodelling advanced by Enlow were confirmed and the growth pattern of the rabbit mandible was elucidated.