Hematoporphyrin, when activated by light, produces lethal effects on lymphoma cells in vivo and in vitro. Mice bearing subcutaneous lymphomatous nodules received hematoporphyrin and two days later were exposed to light. Extensive necrosis occurred within 48 hours after exposure to light. Since no evidence has yet been presented as to what part of the cell is first affected by this treatment, ultrastructural studies were undertaken using an in vitro system. The first morphologic change in leukemic cells occurred within 5 minutes within the mitochondria. First they appeared markedly contracted and subsequently swollen, and ultimately they became completely disrupted. It is hypothesized that possibly mitochondria have a greater affinity for the hematoporphyrin and therefore become the first organelles to be affected
