The exogenously stimulated formation of intracellularly generated Protoporphyrin IX, a precursor of heme, is becoming one of the fastest developing areas in the field of Photodynamic Therapy (PDT). We have examined the degree of protection of several scavengers, aminoacids and compounds related to glutathione metabolism, to the photodamage induced by 5-aminolevulinic acid (ALA)-mediated PDT, employing the LM2 cell line, derived from a mammary murine adenocarcinoma. We have exposed the cells to different concentrations of the scavengers, 24 before PDT, during PDT, and 19 hr after treatment. We defined the protection grade (PG) as the ratio between cell survival after ALA-PDT treatment in the presence of the protector and cell survival after ALA-PDT treatment. We found that L-tryptophan (PG=8.3 at 2mM), N-acetyl-L-cysteine (PG= 7.9 at 30 mM), L-cysteine (PG=7.81 at 8mM), S-adenosyl-L-methionine (PG= 7.86 at 8mM), melatonin (PG=6.81 at 8mM) and glycine (PG=6.8 at 40 mM) are the best protectors to PDT damage, followed by L-methionine (PG=4.38 at 0.8 mM), mannitol (PG=2.32 at 2 mM) and reduced glutathione (PG=3.41 at 0.8 mM), whereas oxidized glutathione does not exert any protection. The implications of these results in the photodamage induced by ALA-PDT is discussed.