The effects of Myleran (1:4-dimethanesulphonyloxybutane) and a series of homologous compounds on the chief types of blood cells in the rat have been investigated. It is possible to give a dose of Myleran which will cause a considerable fall in neutrophils without appreciable effect on red cells or lymphocytes. The effect on the blood of the normal rat appears to give a sufficient indication of the clinical behaviour of Myleran in myeloid leukemia to suggest that it may be a useful guide in the elaboration of new compounds for treatment of this disease. The depressing effect on the neutrophils shown by Myleran and other members of the series may be greater in rats maintained on a low protein diet than in animals maintained on a normal or high protein diet. The toxic effects of this series of components appear to be related almost entirely to the blood changes resulting from effects on the myeloid series. After a fatal dose the animals show a general haemorrhagic state with thrombocytopenia from about the 8th day and death occurs at about the 10th to 12th day as a result of anaemia following a massive haemorrhage usually in the stomach. Myleran is the most active member of the series in causing a fall in blood neutrophils and also shows the greatest selectivity of action in producing a much greater relative depression of neutrophils than of lymphocytes.