1. Contractile activation properties of various types of normal and dystrophic (Duchenne muscular dystrophy, DMD) human muscle fibres were investigated using mechanically skinned fibres activated in Ca2(+)- and Sr2(+)-buffered solutions at room temperature (21-25 degrees C). 2. The majority of the normal human muscle fibres (18/22; 82%) could be classified according to the Ca2(+)- and Sr2(+)-activation characteristics in the same three major groups as other mammalian skeletal muscle fibres studied previously: slow-twitch (8/22; 36%), fast-twitch (3/22; 14%) and fast-twitch intermediate (7/22; 32%), which correspond to the three major histological fibre types I, II B and IIA respectively. 3. Of these three major groups only the slow-twitch (type I; 14/24; 58%) and the fast-twitch intermediate (type IIA; 3/24; 13%) fibres were found in the DMD muscle, indicating that fast-twitch fibres of type IIB were in very low proportion in DMD muscle. 4. The DMD muscle contained a new group of fibres (4/24; 17%) with different Ca2(+)- and Sr2(+)-activation characteristics from the three major histological types. This group of fibres is likely to be of embryonic type. 5. The maximum tension development ability of DMD fibres was less than 20% of that in normal fibres with the exception of some slow-twitch fibres which could produce near normal tension. 6. A significant proportion of normal (4/22; 18%) and DMD (3/24; 13%) fibres were found to have a mixture of Ca2(+)- and Sr2(+)-activation characteristics. This can be explained by co-existence of various myofibrillar protein isoforms in different proportions with regulatory functions in the same individual fibre. 7. The results demonstrate that DMD leads to marked diminution in the ability of most individual skeletal muscle fibres to develop tension, and causes changes in the overall fibre-type distribution in afflicted muscles.