A test procedure for evaluating the cut resistance of yarns under tension-shear loading conditions is described and demonstrated. A knife blade is pressed transversely at a constant rate against a yarn gripped at its ends, the load-deflection relation is measured, and the energy required to cut through the yarn is computed. Results for Zylon (polybenzobisoxazole or PBO) are presented. The cut energy and strain to initiate cutting depend on the sharpness of the blade, the slicing angle, and the pre-tension in the yarn. The dependencies are explained by changes in failure mode of the fibers within the yarn. The test provides information needed to extend a computational model of ballistic response of fabrics to sharp fragments and to design a cutting tool. (C) 2003 Kluwer Academic Publishers.