The effect of various levels of cotton and polyester fibers blended during opening and picking on carding efficiency and processing performance was investigated. The cotton was middling grade, 1 1/16-in. staple length, and 4.2 micronaire reading; the polyester was high modulus, 1.5 in. long, and 1.5 denier. As the percent polyester in the blend increased, compression recovery increased moderately and shear friction appreciably. The increase in friction offset the increase in compression recovery so that cylinder load, fiber hooks, and card sliver variability increased with greater polyester content. However, as the polyester content increased, the increase in fiber friction and length uniformity improved drafting efficiency so that second drawing sliver and yarn uniformity improved. Increasing the polyester content of a blend did not improve yarn grade; high cotton content blends had the same grade as high polyester content blends. At constant end breakage, production in spinning increased rapidly to a 65c/35p blend; additional increases in polyster resulted in only slight improvements in spinning efficiency. Very little polyester was lost in the carding waste even with close flat-to-cylinder settings (0.010 in.), which no doubt provided better carding action on the cotton component in the blend than do settings commonly used. However, it is possible that this close a setting for high percentage polyester blends might cause excessive cylinder loading under mill conditions.