A method is described by which tissue blocks and cell cultures can be rapidly encapsulated for cryoultramicrotomy. Bovine serum albumin, insolubilized with glutaraldehyde, has been found to be a good embedding medium for biological purposes. Ultrathin frozen sections with large fields that are sufficiently uniform for low and high magnification micrographs can be readily obtained. The fine structure of cells and at least some of their enzymatic and chemical properties can be preserved by the described procedures.