RESUMEN
Haematoxylin and eosin [H and E] is the most popular routine stain used in pathology laboratories for highlighting cellular structures. To study the effect of tap water'softening' [i.e. calcium extraction] on H and E stains, 5 sets of slides from 30 different paraffin-embedded human pathologic tissue blocks were prepared in the same way except for washing with 5 different types of water. Slides washed in untreated tap water showed the best results concerning differentiation and colour intensity, while slides washed with softened or other treated water showed poorer degrees of differentiation and colour intensity. The worst results were obtained from slides washed with water containing sodium bicarbonate. Low calcium and magnesium ions and high sodium ions in soft water adversely affect the results of routine H and E stain