THE accepted concept of the action of protective compounds in low-temperature preservation of cells maintains that a small molecular size of the additive is essential to allow its penetration into the cell1,2. A previous investigation by us3, however, demonstrated that polyvinylpyrrolidone (PVP, 30,000 mol. wt.) offers the bone marrow cells good protection (30 per cent) during the freezing and thawing process, being superior to glycerol in this respect. It is obvious that these large molecules cannot enter the cells simply by diffusion. To determine whether the mechanism of this protection involves pinocytosis, hence intracellular penetration, or whether the protective action is extracellular or both extra- and intra-cellular is the object of this investigation.