1. Maximum velocity of shortening (Vmax) and compositions of myosin heavy chain (MHC) and myosin light chain (MLC) isoforms were determined in single fibres from the soleus or the lateral region of the quadriceps (vastus lateralis) muscles in man. Muscle samples were obtained by percutaneous biopsy, and membranes were permeabilized by glycerol treatment (chemical skinning) or by freeze-drying. 2. Types I, IIA and IIB MHCs were resolved from single fibre segments by 6% sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and five different fibre types were identified: fibres containing type I MHC, types I and IIA MHCs, type IIA MHC, types IIA and IIB MHCs, and type IIB MHC. Only a few fibres co-expressed types I and IIA MHCs but 28% of all quadriceps fibres expressed both IIA and IIB MHCs in variable proportions. Fibres co-expressing types I and IIB MHCs were not found. 3. Alkali (MLC1 and MLC3) and dithio nitrobenzoic acid (DTNB) (MLC2) myosin light chains were observed in all type II fibres in variable proportions. MLC (MLC1s and MLC2s) isoforms from type I fibres had lower migration rates than the corresponding isoforms from type II fibres (MLC1f and MLC2f). More than half of type I fibres in both soleus (65%) and quadriceps (68%) muscles also expressed 'fast' MLC3 and 36% of the type II fibres from quadriceps muscle expressed the slow isoform of MLC2. 4. Differences were observed in some mechanical characteristics of freeze-dried versus chemically skinned fibres. Maximum tension (P0) and specific tension were lower in freeze-dried types I and IIA fibres than in chemically skinned, while no differences were observed in the IIA/B fibres. The numbers of types I/IIA and IIB fibres were too low to allow statistical comparisons. In chemically skinned fibres, mean specific tension (0.20 +/- 0.01 N/mm2) did not vary with fibre type. In freeze-dried fibres, on the other hand, specific tensions varied according to MHC type: higher (P < 0.01) specific tensions were observed in types IIB (0.19 +/- 0.01 N/mm2) and type IIA/B fibres (0.18 +/- 0.04 N/mm2) than in type I fibres (0.12 +/- 0.02 N/mm2). The specific tension of type IIA fibres (0.12 +/- 0.05 N/mm2) did not differ significantly from the other fibre types. Cross-sectional areas and mean Vmax did not differ between freeze-dried and chemically skinned fibres, either when all fibres were pooled or within respective fibre types. Vmax data from all fibres of a given type, irrespective of membrane permeabilization technique, have therefore been pooled.(ABSTRACT TRUNCATED AT 400 WORDS)