Protoporphyrin IX (Pp IX) is the main photosensitizer in photochemotherapy with 5-aminolevulinic acid (ALA). Pp IX is photolabile and the present work shows that 70-95% of Pp IX in cells is degraded by clinically relevant light exposures (40-200 J cm(-2) at 630 nm). During light exposure a small yield of photoprotoporphyrin, which is also photolabile, is formed. A substantial fraction of Pp IX in cells incubated with ALA is bound to proteins. During light exposure these binding sites are destroyed, those close to tryptophan residues being the most sensitive. The rate of photodegradation of Pp IX in the cells is dependent on the initial concentration of Pp IX. The degradation mechanisms are therefore not only first order processes. Different degradation rates appear to be related to different types of binding sites. During light exposure, Pp IX molecules appear to move to different binding sites, evidently sites that are more vital for cell survival. Thus, the yield of photoinactivation of the cells, as measured per emitted photon of Pp IX fluorescence, increased during light exposure.