The present invention relates to a phenothiazinium compound of Formula (I): wherein: A and B each independently is 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 XP− is a counteranion and P is 1, 2 or 3; except for the compounds in which A and B are both either —N(CH3)2 or —N(CH2CH3)2 for use in a treatment that requires removal, deactivation or killing of unwanted tissues or cells. The invention also relates to compositions comprising the compounds of Formula I, to selected compounds of Formula I, use of the compounds of Formula I as medicaments and as a PDT agent or a photodiagnostic agent, a conjugate or composite formed between a compound of Formula I and a polymer; and to a method for sterilising fluids in which the fluid is passed over the conjugate or composite whilst it is illuminated.The compounds are biologically active photosensitisers which are strongly photocytotoxic and have application in the areas of photodynamic therapy (PDT), as well as for the diagnosis and detection of medical conditions and related uses in photochemical internalisation, in the production of cancer vaccines, in the treatment and prevention of microbial infections and in photodisinfection or photosterilisation.