The possible effect of cisplatin on porphyrin metabolism was studied in 25 patients with various malignancies treated with high-dose cis-diamminedichloroplatinum. Haematocrit, red blood cells, haemoglobin, white blood cells, platelets and reticulocytes together with coproporphyrin and protoporphyrin in red blood cells were determined before each course of chemotherapy in all patients. In addition, coproporphyrin, uroporphyrin, delta-aminolevulinic acid, and porphobilinogen were determined in the urine just before and 24 h after each course of treatment. Cisplatin administration was followed by a significant suppression of coproporphyrin and protoporphyrin in red blood cells and coproporphyrin, uroporphyrin, delta-aminolevulinic acid and porphobilinogen in urine. The changes observed paralleled similar changes in haematocrit, red blood cells and haemoglobin, strongly suggesting that cisplatin-induced anaemia may be due to a blocking effect of the drug affecting one or more enzymatic steps in the biosynthesis of porphyrins and haem. A moderate fall in the white blood cell count and a mild fall in platelets together with a steady increase of reticulocytes were also observed during treatment.