The photodynamic antibacterial properties of a closely related series of phenothiazinium dyes were tested against several pathogenic strains of Staphylococcus aureus, four of which were methicillin-resistant. Illumination of the photosensitisers at a fluence rate of 1.75 mW cm-2 generally resulted in the enhancement of antibacterial activity in liquid culture and in greater efficacy than the methicillin analogue flucloxacillin. For methylene blue, dimethyl methylene blue and new methylene blue illumination led to increases in bactericidal activity < or = 16-fold, typically 4-fold. In addition dimethyl methylene blue and new methylene blue were active against epidemic strains of methicillin-resistant Staphylococcus aureus at concentrations lower than that of vancomycin (> or = 0.5 microM).