Abstract Background aims. We investigated two different plastic freezing bags, namely the most recently U.S. Food and Drug Administration (FDA)-approved CryoMACS(®) freezing bag (200-074-402) from Miltenyi Biotec and the familiar Cryocyte(®) freezing bag (R4R9955) from (Baxter Healthcare, Deerfield, IL, United States) for the cryogenic storage of human hematopoietic progenitor cells (HPC). Methods. The study material consisted of 12 frozen HPC pairs (= 24 transplant units) that were no longer needed for autologous treatment of patients. After thawing, one unit of a pair was transferred into the Miltenyi (M) bag; the other unit remained in the original Baxter (B) bag. After refreezing both units, all units were stored again under cryogenic conditions either partially immersed in liquid nitrogen (n = 22) or in the vapor phase over liquid nitrogen, n = 2,
