According to the commonly accepted membrane pump theory most of cellular K+ ions are freely dissolved in free cellular water; the alternative association-induction hypothesis postulates that the bulk of cellular K+ is adsorbed (weakly bound) to cellular proteins which are maintained in a specific labile state in the cytoplasm of a living cell. K+ activities measured with ion-sensitive microelectrodes in the cytoplasm of frog skeletal muscle seem to confirm the claim that most of cellular K+ ions are free in cellular water. On the other hand, it is evident from electron microscopic ion binding studies that in frog skeletal muscle most of cellular K+ ions are adsorbed to cellular proteins. The conflicting results can be explained with the assumption that a damage of the cytoplasm caused by the impaling microelectrode leads to a liberation of adsorbed ions. The possibility that microelectrodes damage the muscle cytoplasm is tested by using the light microscope. It is found that microelectrodes produce visible traumas which increase with time. Electron microscopic ion binding studies with damaged muscle support the view that monovalent cations are liberated in the disturbed area of a muscle fiber. It is concluded that a K(+)-sensitive microelectrode is not suited to determine the concentration of free K+ ions in intact frog skeletal muscle.