A mammalian cell suspension prevents pulmonary embolism formation during administration of mammalian cells, such as mammalian stem cells, through a blood vessel, and a preventive agent against pulmonary embolism formation during administration of mammalian cells through a blood vessel. suspending mammalian cells, such as mammalian stem cells, are suspended in a physiological aqueous solution containing trehalose or its derivative, or a salt thereof as an active ingredient to prepare a mammalian cell suspension for preventing pulmonary embolism formation during administration of the mammalian cells through a blood vessel, including the mammalian cells and trehalose or its derivative, or a salt thereof as active ingredients. Examples of the mammalian cells can include pancreatic islet cells, dendritic cells, natural killer cells, alpha/beta T cells, gamma/delta T cells, and cytotoxic T lymphocytes in addition to mammalian stem cells. Preferred mammalian stem cells can include mammalian mesenchymal stem cells and mammalian pluripotent stem cells.