Spectroscopic and physiological studies of the photoinactivation of oral microbes suggest that there is no single mechanism of photodynamic cell kill operating in all microbes, Iris proposed that the photoinactivation of Streptococcus mutans (a Gram-positive bacterium) requires only the generation of O-1(2), whereas the photoinactivation of both Porphyromonas gingivalis (a Gram-negative bacterium) and Candida albicans (a yeast) requires a contribution from another mechanism, most likely a type I electron transfer mechanism involving the photosensitizer. The type I process seems to initiate changes in the permeability of the cell wall of P. gingivalis and C. albicans which permit the sensitizers and probably O-1(2) access to intracellular targets. The hypothesis is supported by new work described here as well as a considerable body of independent data.