This study showed, that in a syngeneic tumour-host system in rats non-sensitive to dextran, dextran 40 and dextran 100 did not stimulate metastasis formation after intravenous tumour cell injection, neither when given as intravenous pretreatment nor when given in the tumour cell suspension. Dextran 1000 stimulated the formation of metastases in both these situations. Disturbed microcirculation, intravascular coagulation and effects upon the tumour cell membrane appear to be the main resons, why dextran 1000 stimulated metastasis formation under the experimental conditions used.