Microorganisms can lead to functional, hygienic and aesthetic (e.g. deterioration, staining) problems on textile products. Natural fibers especially cotton are more easily affected by microorganisms. Blending of cotton fibers with antimicrobial fibers can enhance the protective properties of products against microorganisms. Demand of antimicrobial performance from the products changes depending on the application area. Therefore determination of suitable antimicrobial fiber quantity for the desired application is important. In this study the spinning performance of SeaCell Active/cotton blended open end rotor yarns and antibacterial activities of fabrics produced by these blended yarns were investigated. Five different cotton/SeaCell Active blended slivers with SeaCell Active content from 3 % up to 53 % were prepared on drawframe machine and all slivers were spun into yarns on open end rotor spinning machine at a yarn count of 20 tex with $alpha_{Tt}$=3827 twist coefficient. The effects of rotor speed, opening roller speed, rotor, opening roller and navel type on the quality parameters of SeaCell Active/cotton blended yarns were investigated. Tensile properties, hairiness, unevenness and IPI values of the yarns were reported. All types of cotton/SeaCell Active blended yarns were knitted on a circular knitting machine. Antibacterial activity of the fabrics was analyzed quantitatively. Antibacterial tests showed that good antibacterial activity can be achieved after several washings even with 3 % of SeaCell Active fibers in fabrics.