The distribution and excretion of tritiated actinomycin D have been determined in 3 adult patients with disseminated malignant melanoma. In the blood, the drug was preferentially taken up into nucleated cells. The urinary and fecal excretion was prolonged and only about 30 per cent of the dose of actinomycin was recovered in 9 days. There was evidence that the drug was concentrated in bone marrow and tumor cells, but did not readily cross the blood-brain barrier. The long tissue half-lige of actinomycin D suggeststhat an intermittenr schedule of administration would be the most effective.