The concept of the photodynamic surface is developed. Such surfaces are prepared by incorporating a suitable photosensitizer into a translucent thermoplastic polymer which can for subsequent evaluation and application be presented as a film, fiber, extrudate, molding or bead. The potential applications of such 'photosterile' materials in domestic, medical and industrial situations where it is desirable to maintain microbial populations at a low level are outlined. Polymer containing porphyrin and phthalocyanine sensitizers have been prepared by impregnation (dyeing), codissolution and casting, and copolymerization. Regenerated cellulose impregnated with 5,10,15,20-tetrakis(N- methylpyridinium)porphyrin tetra-p-tosylate has photobactericidal activity against S. aureus, E. coli., P. vulgaris and B. subtilis. The film retains both its mechanical strength and its photobactericidal properties after 50 h exposure to a xenon arc lamp. The effect is photobactericidal and not photobacteriostatic. A potential application in the development of safer domestic wiping cloths has been identified. The mechanisms of the microbicidal effect are considered to be (1) direct cell damage by singlet oxygen generated at the impregnated surface and diffusing to the microbial wall and/or (2) the controlled and slow release of the photosensitizer into solution and the microbial cell.