Abstract— The ability of photodynamic treatment (PDT) with the phthalocyanine Pc 4 to activate cellular signal transduction pathways in murine lymphoma L5178Y-R cells has been assessed by observing increases in protein tyrosine phosphorylation at early times post-PDT. Western blot analysis with an anti-phosphotyrosine antibody revealed a dramatic increase in phosphorylation of two major protein bands of Mr -80000 and -55000 in response to PDT. The increase was PDT dose-dependent, occurred as early as 20 s after initiation of light exposure of Pc 4-pre-loaded cells and was amplified by the presence of the protein tyrosine phosphatase inhibitor, sodium ortho-vanadate (NaV04). By immunoprecipitation, one of the Mr –80000 phosphorylated proteins has been identified as HS1, a substrate of nonreceptor-type protein tyrosine kinases. Although vanadate greatly enhanced the level and extent of PDT-induced phosphorylation, it had no influence on overall photocytotoxicity or on the rate of apoptotic DNA fragmentation. Genistein, an inhibitor of protein tyrosine kinases, diminished tyrosine phosphorylation of the Mr –80000 and other proteins and dramatically potentiated cell killing induced by PDT but did not significantly affect PDT-induced apoptosis. The results suggest that PDT rapidly activates a membrane-associated src family kinase(s) in L5178Y-R cells, one substrate of which is HS1, and that protein tyrosine phosphorylation is part of a stress response, protecting a portion of the cells from the lethal effects of PDT but not altering the mechanism by which they die.