Suspensions ofCandida albicans in saliva, serum, broth or saline were treated with toluidine blue O or methylene blue and exposed to light from a helium/neon orgallium aluminium arsenide laser, respectively, for 120 s and survivors enumerated. Experiments were also carried out in broth at a pH ranging from 4.0 to 7.0. Appreciable kills (>10⁶ cfu ml⁻¹) of the yeast were obtained regardless of the nature of the suspending fluid. Lethal photosensitization of the yeast was also possible at a pH as low as 4.0 Only short-term (3 min) incubatin of the photosensitizer with the yeast was necessary to render it susceptible to killing by laser light.