The preparation of undecalcified bone tissue for electron microscopic investigation depends on the quality of the embedding medium. Penetration of bone tissue is improved by a low viscosity of the monomer plastic material. Hardness of polymerized embedding medium should be similar to bone tissue, but a good cutting quality must be maintained. A low viscosity resin proposed by Spurr (1969) proved to be in compliance with these conditions. The embedding medium is composed of four components: vinylcyclohexenedioxide (ERL, 4206), polyglycolepoxide (DER 736), nonenyl succinic anhydride (NAS) and dimethylaminoethanol (DMAE). The most favourable molar ratio between anhydride (NSA) and epoxide (ERL 4206 and DER 736) was A : E = 0.81 : I. Hardness and brittleness of polymerized epoxy resin can additionally be influenced by varying the amounts of both epoxy components in the final mixture. Human iliac crest trabecular bone as well as rat bone tissue were embedded to test the quality of the medium. Resutls: Ultra-thin sections were performed at low cutting speed (0.5 mm/sec) using diamond knives. Artificial alterations of bone surfaces did hardly occur due to a good penetration of the bone tissue. The special hardness of the embedding medium stabilized the tissue, thus chattering of the transition from osteoid to mineralized bone could be markedly reduced.