Propylene oxide (1,2-epoxy propane) has long been utilized as a transitional solvent in the preparation of biological tissues. This compound compound is completely miscible with the various epoxy resins utilized for infiltrating tissues. Ethanol, perhaps the most widely used dehydration agent, does not possess this characteristic and must be removed before the process of infiltration commences. It is common practice amongst biological microscopists to pass tissues for a few minutes through pure propylene oxide (PO) immediately prior to infiltration. The tissues then are pre-infiltrated with various proportions of PO and the embedding medium of choice. For example, one traditional recipe takes the tissues progressively from pure PO to 2:1 PO/resin, 1:1 PO/resin, and 1:2 PO/resin mixtures, with stays of a few minutes at eacil stage. A lengthy procedure such as tills probably is not necessary and has never been favored by this worker.