Abstract: A method for treating yarn in package form with a yarn treating agent to modify the physical and/or chemical characteristics of the yarn comprising the steps of introducing the agent at selected portions of the outer surface of the package, and then infusing the agent into the package with a heated fluid applied under pressure. More particularly, the method of the present invention contemplates the treatment of yarn in package form with one or more dyes or dye acceptance modifiers to modify or dye the yarn fibers in a reproducible manner to provide repeating contiguous sections of yarn having the desired characteristics. In one preferred embodiment of the invention, a dye acceptance modifier in the form of a resist is infused into one or both of the ends of a yarn package and a heated fluid is then introduced under pressure at a temperature and for a time sufficient to cause the resist to partially penetrate the package and contact individual strands of yarn or fibers, and to set the resist, if necessary.