Normal human red blood cells, when exposed briefly to acetylphenylhydrazine (APH), acquire Heinz bodies and a propensity for net ion and water loss upon subsequent incubation in an APH-free medium of physiologic sodium and potassium (K) content. The cells can be protected from APH damage by previous deoxygenation. The ion and water loss depend on the presence of a K gradient from cell to medium. In contradistinction to some other types of membrane perturbation in which K permeability is increased, the APH effect is not dependent on calcium. The meaning of these observations is discussed in relation to the vulnerability of the K permeability barrier.