Collins and Edwards (1971, Am. J. Physiol., 221:1130-1133) have shown that a tissue potential can be measured with microelectrodes in glycerinated muscle and that this potential is consistent with a Donnan equilibrium of small ions due to the concentration of net fixed electric charge on the contractile proteins. This approach has been combined with x-ray and light diffraction measurements of the muscle lattice dimensions, and the data are used to determine the thick filament charge and thin filament charge under a variety of different conditions. The thick filament charge is a function of the bathing solution, in particular its pH and ionic composition. These parameters are important in determining the volume of the equilibrium lattice and possibly are involved in the contraction mechanism itself.