Abstract: The present invention provides a process for the production of thin sections of biological tissue, especially for transmission electron microscopy. In the process a fixed tissue sample to be investigated is dehydrated by infiltration of a hardenable medium miscible with water and subsequently embedded in casting resin, whereafter thin sections are cut from the cast body resulting after hardening of the casting resin. Infiltration is carried out by warming the fixed tissue sample in a mixture of formaldehyde, melamine and water. During warming, the mixture forms water soluble monomeric methylolmelamines which infiltrate the tissue and undergo polycondensation to form water-insoluble polymeric methylolmelamine condensates in the tissue. The casting resin is preferably a melamine resin, and the resin is preferably prepared using a methylolmelamine ether solution containing polyethylene glycol.