Using two-dimensional electrophoresis we investigated the effect of 5-aminolevulinic acid (ALA)–based photodynamic therapy (PDT; induction with 1 mM ALA for 4 h followed by blue light dose of 18 J/cm²) on the protein expression in HL60 leukemia cells. ALA-PDT resulted in extensive qualitative and quantitative changes in the protein pattern of HL60 cell lysates. Of more than 1350 protein spots recognized on the protein maps of ALA-induced cells, seven proteins were enhanced and 17 suppressed following irradiation. Three of these, calreticulin presursor, p58 microsomal protein (ERp57) and protein disulfide isomerase (p55) have been identified by matrix-assisted laser desorption and ionization-mass spectrometry and the pI/molecular weight parameters of the affected proteins were estimated by computer analysis. The findings suggest participation of endoplasmic reticulum Ca² -binding chaperones and/or Ca² signaling in ALA-PDT mediated cytotoxicity.