Submaximum Ca2+- and Sr2+-activated tensions were measured in functionally skinned fibres from the rabbit diaphragm. 2. The fibres could be classified into two types on the basis of activation by divalent cations. Type S fibres required a slightly higher [Ca2+] than [Sr2+] for half-maximal activation of tension. Type F fibres required 2 times higher [Ca2+] than type S and 8.5 times higher [Sr2+]. 3. Sodium dodecyl sulphate disc gels of type F fibres showed protein band patterns similar to those of fast-twitch muscle; those of type S fibres were identical to those of slow-twitch fibres. 4. Sodium dodecyl sulphate disk gels of mixed, non-classified diaphragm fibres and an actual count of randomly selected fibres classified by Ca2+/Sr2+ tension characteristics showed the diaphragm to be composed of 60% fast-twitch fibres and 40% slow-twitch fibres. 5. The protein stoichiometries of the two fibre types are consonant with the accepted stoichiometries for skeletal muscle. 6. The percentages of type F and type S fibres in the diaphragm suggest that high adenosine-triphosphatase activity shown by histochemical techniques (Davies & Gunn, 1972) is associated with the type F fibres, and low activity with the type S fibres.
