The commonly used procedures for in situ hybridization require treatment of the tissue with non-ionic detergents and proteolytic enzymes, resulting in considerable loss of morphological detail. In this study proteinase pre-treatment of the tissue was replaced by saponin, a highly surface-active plant glycoside. This saponin treatment allowed good preservation of tissue morphology, as determined by differential interference and contrast enhanced video microscopy. Saponin pre-treatment resulted in an equal or even better hybridization sensitivity with probes recognizing viral (canine distemper virus) and cellular (myelin) nucleic acid sequences in tissue cultures as well as in paraffin sections. Probable mechanisms of how saponin allows probe penetration while maintaining the morphological details are discussed.