A new method has been developed to detect RNA-DNA hybrids in situ by fluorescence microscopy. This overcomes some of the disadvantages of autoradiographical detection of in situ hybridization, notably the low resolution and long exposure times needed. A procedure to label RNA at its 3'-terminus with a fluorochrome molecule has been developed. The optimal conditions for the cytochemical hybridisation reaction of this fluorochrome-labeled RNA were investigated using a model consisting of Sepharose beads to which nucleic acids has been bound. With RNA labeled both with 3H and rhodamine the hybridization reaction could be studied both biochemically by scintillation counting and cytochemically by microfluorometry. The fluorochrome-RNA bond was found to be unstable at higher temperatures. Therefore, the hybridization reaction had to be performed at room temperature in formamide-containing buffers. With fluorochrome-labeled complementary RNA kinetoplast DNA in Crithidia luciliae, adenovirus-5 DNA in infected KB cells and 5S rRNA, tRNA and cloned histone genes in polytene chromosomes of Drosophila could be localised. An immunocytochemical amplification method was developed that increased the sensitivity of the direct method. Several recently developed hybridocytochemical methods based on a combination of a hybridisation reaction in the first layer followed by immunocytochemical second layers, are described. The prospects of the application of the hybridocytochemical techniques in biomedical research, such as gene localisation and virus diagnosis, are discussed.