The results of an irradiation of sensitized and non-sensitized tumors Pliss lymphosarcoma are presented in the paper. The
experimental data and the theoretical calculations show, that hemoglobin can be considered as a primary agent, the
photoexcitation of which induces cytotoxic reactions resulting in a tumors destruction without an exogenous singlet
oxygen photosensitizer. Several mechanisms of a laser action on biological tissues, which can hypothetically influence
on PDT efficiency, are discussed in the paper.