Benzoporphyrin derivative monoacid ring A (BPD) is taken up rapidly (within 30 minutes) by most cells. Rapidly dividing tumor cell lines and mitogen activated murine T lymphocytes were found to take up significantly more (5 - 10 fold) BPD than do normal splenic lymphocytes making them a potential PDT target. Experiments have shown that BPD can be activated in the blood of animals by whole body irradiation with red light, shortly after intravenous administration, in the absence of skin photosensitivity. During the treatment time, plasma levels of BPD were between 0.7 and 1.0 (mu) g/mL. The light treatment resulted in between 70 and 80% photoinactivation of circulating BPD. When L1210 tumor cells were preincubated with BPD and injected i.v. into mice immediately before total body light treatment, significant reductions in circulating clonogenic tumor cells were observed in blood samples taken immediately following treatment. This `transdermal' treatment has been shown to be effective in preventing the development of hind limb arthritis in MRL/1pr mice.