The biodistribution of a new and very potent photosensitizer, benzoporphyrin derivative-monoacid, ring A (BPD-MA), was determined in normal and P815 (mastocytoma) or M1 (rhabdomyosarcoma) tumor-bearing DBA/2J mice. A dose of 80 micrograms of 3H-BPD-MA was determined at 3, 24, 48, 72, 96 and 168 h post injection. The following tissues were tested: blood, brain, heart, intestine, kidney, lung, liver, muscle, skin, stomach, spleen, thymus and tumor. The biodistribution of 3H-BPD-MA in normal and tumor-bearing mice was comparable overall. 3H-BPD-MA localized in tumors better than in other tissues except kidney, liver and spleen. The tumor to tissue ratios were in the range 1.5-3 at 24 h post injection and increased further during the next 72 h. The highest levels of 3H-BPD-MA were observed in all tissues at 3 h post injection and decreased rapidly during the first 24 h. After 24 h the clearance from tissues was rather slow. The preliminary clearance data obtained in a group of five normal mice indicated that the majority of the injected dose (60%) cleared from the body via the bile and feces, while only about 4% cleared via kidneys and urine. Studies in which 3H-BPD-MA was extracted from tumor, kidney and liver 3 and 24 h after injection showed that, at 3 h, all the photosensitizing activity in tumor was retained. At 24 h only 39% of the activity was retained and considerably less active material was present in liver and kidney.