HeLa cells and synovial tissue sections were used for visualizing poly A+-mRNA after in situ hybridization. We describe a method to biotinylate hybridization probes by tailing oligo-dT nucleotides with biotinyl-dUTP using terminal deoxynucleotidyl transferase (E.C.2.7.7.3.1.). The hybrids were detected by using the strepavidin-biotin-peroxidase complex (ABC method). The reaction product of the peroxidase staining in the cells and in the tissue sections, is quantitated by the method of plug- and scanning photometry respectively.