Biopsies of erector spinae (sacrospinalis) and multifidus muscles were taken, in a precise manner, adjacent to lumbar vertebra 5, from 18 patients aged 28-73, suffering from acute lumbar disc disorders. The respective muscle fiber characteristics were assessed histologically and histochemically using ATPase and NADH staining procedures. A 'strength factor' component was developed for each muscle by combining measurements of fiber area and fiber type, to introduce a functional indication of force. No obvious signs of muscle pathology were evident histologically or histochemically. While a large difference in muscle characteristics may exist on opposite sides of the vertebral column in individual subjects, these differences do not appear to be related to the side of the disc protrusion. It is therefore suggested that muscle obtained from patients suffering from disc protrusion can be used as representative of normal muscle.