The object of the present invention is to provide a cell mass or cell structure-embedding agent that can stably transport cell masses or cell structures in a case of transportation at a low temperature and after transportation, conveniently recover a cell mass or cell structure from the cell mass or cell structure-embedding agent, a cell mass or cell structure-containing composition, and a kit. According to the present invention, provided is a cell mass or cell structure-embedding agent including: polypeptide which is represented by Formula 1 and in which an area of the maximum molecular weight peak in molecular weight distribution measurement is 80% or more of the total area of all of the molecular weight peaks.A-[(Gly-X-Y)n]m-B ； ；Formula (1):In the formula, X's and Y's each independently represent an amino acid, m is an integer of 2 to 10, n is an integer of 3 to 100, and A and B represent any amino acids or amino acid sequences.
