To provide a therapeutic agent for instant blood-mediated inflammatory reaction (IBMIR) caused by blood contact with cells to be transplanted after they are transplanted into a foreign recipient.

The therapeutic agent for IBMIR contains a dextran sulfate ester or a pharmaceutically permissible derivative thereof and is useful for treating morphological degradation of cell transplantation and histoincompatibility of cell transplantation caused by IBMIR, provided that Langerhans islets of the pancreas are excepted.