Although photodynamic therapy is being used increasingly for the treatment of cancer, its effect on immune responses has received little attention. This aspect was studied in a rat model. Rats were given (intravenously) lymph node cells which were simultaneously exposed to a photosensitizer and light. As a result of this treatment the animals showed a decreased contact hypersensitivity response. Most important is that the generated immunosuppression proved to be non-specific. The consequence of this finding may be that photodynamic therapy of tumours can inhibit the immune response against malignant cells.