The exogenously stimulated formation of intracellularly generated protoporphyrin IX, a precursor of haem, is becoming one of the fastest developing areas in the field of photodynamic therapy (PDT). We tested the action of several free radical scavengers, amino acids, antioxidants and sulphur-containing compounds as protectors from photodamage induced by 5-aminolaevulinic acid (ALA)-mediated PDT, employing the LM2 cell line, derived from a mammary murine adenocarcinoma. We exposed the cells to different concentrations of the compounds, 24 h before PDT, during PDT, and 19 h 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 of ALA-PDT treatment alone. We found that l -tryptophan (PG=9.2 at 2 m m ), reduced glutathione (GSH) (PG=5.8 at 0.8 m m ), N-acetyl- l -cysteine (PG=4.86 at 30 m m ), melatonin (PG=4.5 at 8 m m ) and l -methionine (PG=4.0 at 0.8 m m ) are the best protectors from PDT damage, followed by l -cysteine (PG=2.8 at 0.8 m m ), mannitol (PG=2.6 at 20 m m ) and glycine (PG=2.4 at 40 m m ) whereas oxidised glutathione and S-adenosyl- l -methionine do not exert any protection. We did not found any photoactive action of the protectors in absence of ALA. These results can be considered to modulate the photodamage induced by ALA-PDT.