Mercox is a methylmethacrylate-based resin which is widely used for vascular corrosion casting with subsequent scanning electron microscopic analysis. In the present study the effect of undiluted and diluted Mercox (4 + 1; volume + volume; Mercox: monomeric methyl-methacrylate (MMA); 0.02 g catalyst MA/ml Mercox) and methyl-methacrylate with and without catalyst MA (0.625 g/10 ml MMA) on fixed and unfixed endothelial cells was studied. Light microscopy (LM) of cultured capillary endothelial cells (ECs), which were replicated with diluted or undiluted Mercox shows degranulation and membrane perturbation of ECs, while no morphological changes occur in glutaraldehyde-prefixed ECs. Scanning electron microscopy (SEM) of replicas (= resin blocks) polymerized on prefixed ECs reveals unchanged ECs and replicas show many details. Unfixed ECs are destroyed and replicas reveal aberrant features. Transmission electron microscopy (TEM) of prefixed and unfixed ECs (cultured endothelial cells, endothelial cells of perfusion prefixed and of unfixed tadpole tail fin vessels) substantiates LM and SEM findings. Prefixed ECs resist Mercox without fine structural changes, while unfixed cells undergo destruction. It is recommended to fix vessels prior to casting. Extravasations in micro-vessels are considered to be caused by focal chemical destruction of endothelial cells.