An important requirement for valid immunohistological assays is to have adequate test material, i.e. the tissue sample must be representative of the organ (and the lesion to be studied) and must be prepared for preservation of both morphological detail and antigenicity. Hence, tissue sampling needs critical considerations in studies on structure-function relationships. It appears quite dull, but one has always to realize that fixations, dehydrations, embedments etc. which usually proceed histological staining will cause various changes of the natural specimen. With respect to reliable interpretation, one needs knowledge about the possible changes of biomolecules caused by tissue preparation.