This invention addresses the problem of providing: a mammalian cell suspension capable of preventing the formation of pulmonary embolisms when mammalian stem cells or other mammalian cells are administered intravenously； or a prophylactic agent against the formation of pulmonary embolisms when mammalian cells are administered intravenously. This invention is characterized by the suspension of mammalian stem cells or other mammalian cells in a physiological aqueous solution including as an effective component thereof trehalose, a derivative thereof, or a salt of either, and the preparation of a mammalian cell suspension for the prevention of pulmonary embolism formation when administering mammalian cells intravenously, said suspension containing as an effective component thereof: mammalian cells； and trehalose, a derivative thereof, or a salt of either. In addition to mammalian stem cells, islet cells, dendritic cells, natural killer cells, alpha-beta T cells, gamma-delta T cells, and cytotoxic T lymphocytes, etc., can be used as the mammalian cells. Mammalian mesenchymal stem cells and mammalian pluripotent stem cells are suitable examples of mammalian stem cells.
