In the present paper it was appraised the potential of two São Paulo cotton varieties (distributed for planting in the State of São Paulo, Brazil) and the effect of its raw material blended with sintetic fiber represented by polyester, concerning to the main characteristics of yarn obtained by the new process known Open-End Spinning. The cotton utilized in thus study, came from IAC 17 and IAC 18 varieties, which were harvested at the Experimental Stations of Instituto Agronômico, situated in Tiete and Tatuí, respectively during the 1978/79 growing season. With the IAC 18 variety was obtained Open-End yarn of the best quality when compared with IAC 17 variety. The differences on yarn characteristics were not so marking as those founded in physics properties of fiber. The blend of polyester to cotton utilized, improved the characteristics of the yarn, mainly in relation to tenacity. The spinning industries that utilize this process, can forecast the quality of yarns to be produced by using the cotton studied, or blends with polyester, through the regression equations presented.