In human endothelial cells ECV 304 and HMEC-1 photosensitized by pyropheophorbide-a methylester (PPME) in sublethal conditions transcription factor Nuclear Factor kappa B (NF-κB) activation takes place for several hours. Activated NF-κB was functional because it stimulated the transcriptional activation of either a transfected reporter gene or the endogenous gene encoding interleukin (IL)-8. Concomitant with NF-κB activation, inhibitor of NF-κBα (IκBα) was degraded during photosensitization and IκBβ, p100, p105 and IκBε were slightly modified. Reactive oxygen species (ROS) were shown to be crucial intermediates in the activation because antioxidants strongly decreased NF-κB activation. Using both a fluorescent probe and isotope substitution, it was shown that ROS, and especially singlet oxygen (¹O₂), were important in the activation process. Because NF-κB activation in the presence of ROS was suspected to proceed through a pathway independent of the IκB kinases (IKK), we demonstrated that the IKK were indeed not activated by photosensitization but required an intact tyrosine residue at position 42 on IκBα, suggesting the involvement of a tyrosine kinase in the activation process. This was further reinforced by the demonstration that herbimycin A, a tyrosine kinase inhibitor, prevented NF-κB activation by photosensitization but not by TNFα, a cytokine known to activate NF-κB through an IKK-dependent mechanism.