The present invention provides a technique which enables organization of bone marrow cells by a simple method in a short period of time. A method for preparing a bone marrow cell aggregate, comprising adding a liquid containing a bone marrow cell population to a medium containing a swellable material and culturing the bone marrow cell population in the presence of the swellable material. A method for reassembling a bone marrow tissue, comprising adding a liquid containing a bone marrow cell population to a medium containing a swellable material and culturing the bone marrow cell population in the presence of the swellable material. According to common knowledge in the art, it has been considered difficult to reorganize once disintegrated bone marrow tissue without changing the cell composition (that is, without adding any adherent cell or extracellular matrix which will work as a "connecting material (binder)"). Indeed, it was impossible to aggregate bone marrow cells by conventional methods. As a result of
