Benzoporphyrin derivative monoacid ring A [BPD-MA (verteporfin) or BPD], a second generation photosensitizer tested in clinical trials in combination with red light was compared for its PDT efficiency in vitro and in vivo upon activation with light in the UVA, blue and red spectral ranges. PDT efficiency, calculated based on the BPD absorption spectrum and spectral output of the different light sources, was compared with actual PDT efficiency determined in vitro and in vivo. Results obtained in an in vitro cytotoxicity assay, in which aliquots of murine P815 cells, pre-incubated for 1 h with BPD at 5 ng/mL, were exposed simultaneously to various light doses delivered within UVA, blue and red spectral ranges showed that in this test system PDT efficiency was governed by BPD absorption and light source emission spectra. Similar results were obtained in an in vitro BPD photobleaching test. Thus in vitro, values for calculated, theoretical PDT efficiency corresponded to the actual PDT efficiency. However, in vivo factors, such as depth of tissue penetration with light and localization of the target, had an important influence on PDT efficiency. In mouse models of skin photosensitivity and the cutaneous hypersensitivity immune response (CHS) assay, because of the thinness of mouse skin, PDT efficiency approximated the theoretical PDT efficiency, although blue light was somewhat more efficient in PDT than UVA, and red light was somewhat more efficient than blue or UVA. In a pig skin photosensitivity model, red light induced the highest skin response manifested by erythema and swelling, while blue light caused erythema and minimal swelling and UVA caused only erythema. These differences could be related to the thickness of pig skin and the depth of tissue penetration characteristic of each spectral range. Fluence rate was found to be an additional factor which modifies the effect of BPD and light. In conclusion, BPD can be efficiently activated with light within the UVA, blue and red spectral ranges. Moreover, light doses, deemed safe for red light, can be utilized with light of other spectral ranges, but only after a very careful evaluation of the conditions under which they were determined and the conditions under which they will be used.