Methyl cellulose syndrome induced by repeated i. p. injections of methyl cellulose (MC) was followed in experimental rats. Following application of 2--4 MC injections early haematological changes are observed including increased values of reticulocytes, development of serum aggregation factor having anti-erythrocyte antibody nature and altered filtrability of non-washed erythrocytes. Later alterations developing with further MC injections (8-32) are characterized by expressive splenic enlargement, by decrease in erythrocyte and platelet values and by additional increase in number of reticulocytes. The cause of anaemia is pooling and sequestration of erythrocytes in the spleen and haemodilution from hypervolaemia blood plasma. The decreased platelet amount is the result of reduced survival time of platelets due to their increased sequestration in the spleen. Haematological changes are normalized after splenectomy. This picture resembles to a great extent human hypersplenism.