Total collagen, heat soluble collagen, fat, moisture, sarcomere length and pH were determined on 18 muscles from two commercial beef animals. Each muscle was given a mild, moderate or severe cooking and the toughness determined using a mechanical compressive test. From the force-deformation curve, measured throughout the test, three toughness values: the yield force, the final compressive force and the work done, were taken. In general, collagen content was associated with the compressive force and the other components with the yield force, although no precise distinction could be made. Collagen content, in particular the amount of total collagen, was the best predictor of toughness, but as thecooking becamemore severe, collagen accounted for progressively less of the variability in toughness. The other components contributed substantially to the variability in toughness only when meat was moderately cooked.