A phenothiazinium compound of Formula (I): ##STR00001## wherein: A and B each independently is ##STR00002## in which R' and R'' each independently is a linear, branched or cyclic hydrocarbon group, or R' and R'' together with the N atom to which they are attached form an optionally substituted 5-, 6- or 7-membered ring; and where X.sup.P- is a counteranion and P is 1, 2 or 3; except for the compounds in which A and B are both either --N(CH.sub.3).sub.2 or --N(CH.sub.2CH.sub.3).sub.2 for use in a treatment that requires removal, deactivation or killing of unwanted tissues or cells.