Blended textiles of wool and cotton are conventionally dyed to one uniform shade in one or two dyebaths with one or two dyes where the different fiber types are dyed sequentially in two steps, under different requirements for alkalinity/ acidity and temperature. This cumbersome process has now been simplified to a one-step dyeing process with one dye. The new process relies on the selective modification of the cotton component in the presence of wool in a pretreatment step to dyeing, followed by dyeing from a fresh dyebath with a wool or cotton dye. The pretreatment compounds are commercially available dye fixatives that impart cationic character and polyamino functionality to cotton cellulose. These compounds vary in their ability to convey union dyeing behavior to wool/cotton blends. The new pretreatment/ union dyeing systems are dependent on processing conditions for the quality of union shade. Dye fixatives of various functions were used with various dye classes to determine the optimum conditions for achieving union shades of high color strength and colorfastness. The most effective pretreatment/union dyeing system was found to be pretreatment with a poly(diethylenetriaminebiguanide) (PDETAB) compound, followed by dyeing with a wool reactive dye with alpha-bromoacrylamido functionality. This report describes the effectiveness of PDETAB in promoting union dyeing of wool/cotton blended fabrics, including cross-wovens (interlacing wool and cotton yarns), intimate yarn blended fabric of wool and cotton, and wool and cotton fibers before blending.