SUMMARY: The time course of lysis of protoplasts of Micrococcus lysodeikticus was studied by spectrophotometric methods. The lytic agents used were sodium dodecylsulphate and a range of cationic detergents which each possessed a n-C12 alkyl chain and comprised the amine hydrochloride, the trimethylammonium bromide, the pyridinium bromide and the quinolinium bromide. Also the inhibition of lysis by uranyl ions and the action of detergents on isolated protoplast membranes were investigated. It is concluded that the cationic detergents act on the complex phosphatidic acid lipid component of the protoplast membrane and that lysis results from secondary osmotic effects. Access to the reactive site by agents of similar hydrophilic-hydrophobic balance is controlled by steric factors. Anionic detergents, however, cause direct disintegration of the protoplast, membrane independent of osmotic stresses, probably by interaction with the membrane protein moiety; a concurrent interaction with lipid, possibly through a lipoprotein complex, is not definitely excluded.