The present invention addresses the problem of providing a disease-treating agent which exerts an excellent effect in treating diseases requiring an emergency surgery such as heart failure and which is efficacious at a certain level for a large number of patients. The present invention pertains to a kit for preparing a disease-treating agent, said kit comprising a) a fibrinogen solution, b) a thrombin solution and c) mesenchymal stem cells, in separate forms respectively. It is preferred that c) the mesenchymal stem cells are allogeneic to a subject to be treated. Also, the present invention pertains to a kit for preparing a disease-treating agent, said kit being to be used by, when in use, suspending c) the mesenchymal stem cells in either a) the fibrinogen solution or b) the thrombin solution, and then spraying the cell suspension thus obtained directly to a disease site substantially simultaneously with either b) the thrombin solution or a) the fibrinogen solution that is not used in the suspending step.
