Summary In rats a maximal tolerated dose of carboplatin (60mg kg-1, i.v.) caused severe anaemia, leucopenia and thrombocytopenia. These indices of haematological toxicity were also observed with a maximal tolerated dose of cis-platin (6.5mgkg-1, i.v.), but reductions in blood cell counts were less than those observed with carboplatin. Anaemia was deduced to be the dose-limiting toxicity of carboplatin, since red cell transfusions afforded protection to rats receiving a lethal dose of this compound (80mgkg-1, i.v.). Anaemia did not appear to be due to an increase in the susceptibility of cis-platin-or carboplatin-exposed red cells to lysis, as concluded from results of osmotic fragility tests. These red cells, when tagged with 51Cr, also did not exhibit reductions in survival time. Administration of 5 Cr-labelled control red cells to rats, which had been treated with carboplatin 3 days earlier, resulted in substantial loss of the radiolabel from the circulation, indicating that internal haemorrhaging, as a result of thrombocytopenia, is probably the principle cause of drug-induced anaemia.