All living cells suspended in aqueous salt solutions at neutral pH values possess a negative charge. The charge is due to the dissociation of ionogenic, or charged, groups (carboxyl, amino, and others) in the cell surface. Studies of the charge carried by bacteria give information regarding the nature of the components in their surfaces; these studies have been used to measure the frequency distribution of bacterial variants in a mixed population and can assist in the selection of material for vaccine production. The occurrence and nature of tumour cells and the selection of yeasts for brewing have been studied electrophoretically.