Dextran infusion causes an increased lysability of ex vivo thrombi. This increased lysability is shown to be accompanied by a change in the distribution of51Cr-labelled platelets. Rather than being localized to the head of the thrombus, after dextran infusion the platelets are found to be more evenly distributed. This change in platelet distribution is secondary to an effect of dextran on platelet function, probably via previously demonstrated impairment of the function of factor VIII. It is suggested that the increase in thrombus lysability is due to the change in platelet function. © 1979 S. Karger AG, Basel.