In this work the possibility and potential of treating cotton fibers and yarns instead of fabrics with monocomponent cellulases was investigated. Different pretreatments on fibers were performed and tested in order to improve the accessibility of cotton to enzymatic modification. The enzymatic treatments were evaluated microscopically and by analysing the effects of treated fibers on spinnability, yarn evenness, tenacity and pilling. The accessibility of the cotton fibers for cellulases could be increased by different pretreatments. Steaming of fibers prior to enzymatic treatment was found to be an efficient way to increase hydrolysis levels. Cellulase treatments of carded yarns resulted in modification of yarn properties. Decrease in yarn hairiness was observed and the knitted fabric made of the treated yarn showed a lowered tendency towards pilling. In all cases endoglucanase activity rather than cellobiohydrolase activity was responsible for these modifications.