NB Formaldehyde concentrated 5X enables fast and economical preparation of NB solution of 4% formaldehyde (10% formalin). The prepared solution is stable at low temperatures, it causes no precipitates. It is suitable for fixating bioptic materials, smaller tissue samples and long-term fixated samples storing. It is a colorless solution with a characteristic odor, ready for applying. Tissue samples can be fixated and/or stored for a longer period of time without severe hardening of tissue or formalin pigment forming in tissues, which then leads to process of removing the so-called acid hematein. It is suitable for usage in all automated devices for tissue processing, as well as for manual histological techniques.