A revised technique for preparing human tissues and organs aiming at application to medical use is provided. With this technique, function per cell is greatly improved and great reduction in production cost is realized. A method of preparing an organ bud, comprising culturing vascular endothelial cells, mesenchymal cells and a tissue or organ cell in vitro in the presence of blood cells. Also provided are an organ bud prepared by the above method and a method of preparing a tissue or an organ using the organ bud. Using the same organ bud, there are provided a method of transplanting an organ bud, a method of regeneration or function recovery of a tissue or an organ, a method of preparing a non-human chimeric animal, and a method of evaluating a drug.
