The distribution of histochemically identified muscle fibre types was studied in biopsy samples from the two main muscles in the lumbar region of the human erector spinae, the multifidus and the longissimus, in 16 healthy subjects (nine males and seven females, age 20-30 years). Muscle fibres were classified as types I, IIA, IIB or IIC on the basis of the pH lability of their myofibrillar ATPases. There were no differences between the multifidus and the longissimus muscles in the relative occurrence of type I (62 vs. 57%), type IIA (20 vs. 22%) or type IIB fibres (18 vs. 22%), or in the absolute size of fibres (range of mean least diameters 58-66 micron). The oxidative potential (NADH-diaphorase staining intensity) was high in type I and low in type II fibres, irrespective of subgroups, in both muscles. In the females, the type I fibres occupied a relatively larger area (70-75 vs. 54-58% for the males) although the relative number of type I fibres was the same in both sexes. This was due to smaller type II fibres in the females resulting in higher type I/type II area ratios (1.70-1.90 vs. 0.88-0.92 for males). This suggests a difference in functional capacity of lumbar back muscles between males and females. On the other hand, the similarity in histochemical fibre-type distribution between the multifidus and the longissimus muscles does not give support for a functional differentiation between these two anatomically different parts of the lumbar erector spinae in man.