The in situ reverse transcription-polymerase chain reaction (in situ RT-PCR) method has been used to detect low levels of expression of mRNA in cells and tissues. We applied a newly developed in situ RT-PCR to cells and quantitatively compared the sensitivity of the in situ RT-PCR with that of conventional in situ hybridization (ISH). Human tumor cell lines with either high, moderate, low or no expression of IL-6 mRNA as determined by in vitro competitive RT-PCR were used. In situ RT-PCR was performed using Thermos thermophilus DNA polymerase, special primer sets to form high molecular weight concatemers and an in situ digoxigenin (DIG) direct labeling technique. Conventional ISH was performed using DIG-labeled cRNA probe. Specific signals of IL-6 mRNA were observed in the cytoplasm of positive cells by both in situ RT-PCR and ISH. in situ RT-PCR was more than 100-fold more sensitive than ISH. The specificity of in situ RT-PCR was confirmed by appropriate control tests. The data indicated that the sensitivity of our in situ RT-PCR applied to culture cells was significantly higher than that of ISH, and that the technique retained high specificity.