An overview is provided concerning the preparation and use of radiolabelled single-stranded RNA probes (riboprobes) for in situ hybridization. All theoretical aspects are covered as well as some practical examples from the authors' experience. The use of riboprobes for in situ hybridization represents a valid and highly sensitive alternative to oligonucleotide probes. A sample protocol is finally described in detail: this should enable any investigators to design the ISH assay that best fits their particular needs.