Detection of endogenous messenger RNA in surgical pathology tissue samples is a useful technique for identifying or characterizing populations of normal or neoplastic cells. Potential problems with this technique include degradation of target RNA in routinely processed specimens, and low concentration of target RNA sequences in the cells, necessitating the use of radiolabeled probes for detection. In this report, we demonstrate detection of nonmuscle actin messenger RNA in fixed, paraffin-embedded lymphoid tissues and bone marrow aspirate smears, using routinely obtained surgical and hematologic specimens. In addition, increased sensitivity was achieved by hybridization with full-length RNA probes, rather than oligonucleotide DNA probes; these probes were labeled with digoxigenin-11-UTP, allowing colorimetric detection of hybridization and avoiding the use of isotopes. The advantages and potential applications of this technique as an aid in pathologic diagnosis are discussed.