We and others have described methods to label specific nucleic acid sequences in fixed cells by reverse in situ transcription (IST). They are simple alternatives to the tedious steps of in situ hybridization with labeled probes. We have favored use of thermostable DNA polymerases after heat denaturation of template secondary structure, accompanied by synthesis of cDNA from an annealed primer, but the approach has been limited by the low reverse transcriptase (RT) activity of Taq polymerase and delayed detection methods. We have improved the technique by the use of recombinant Thermus thermophilus (rTth) DNA polymerase and fluorescein-12-dUTP (FIST). Jurkat T lymphocytes were stimulated with ionomycin + phorbol myristate acetate to produce interleukin-2 (IL-2) mRNA in vitro overnight. They were cytospun onto slides and fixed in 70% ethanol + 30% DEPC-treated water, acetone, and air-dried. The slides were placed on a temperature-controlled heating block, and the cell spot was covered with a plastic coverslip. The temperature was raised to 95 degrees C, and 5-10 microliters of modified Perkin-Elmer/Cetus rTth RT reaction mix was injected under the edge of the coverslip. Each 10 microliters of mix in DEPC-water contained 10 mM Tris-HCl, pH 8.3, 90 mM KCl, 1 mM MnCl2, 1 mM dithiothreitol, 10 U placental ribonuclease inhibitor, 0.125 mM dA,C,GTPs, 0.1 mM fluorescein-12-dUTP, 2 U rTth DNA polymerase, and 4 pM 22-mer oligonucleotide primer, which spanned the second intron of IL-2. After 3 min at 95 degrees C, 1 min at 50 degrees C and 10 min at 72 degrees C, the slides were washed in 0.5 x phosphate-buffered saline, pH 7.0, at 42 degrees C, in 70% ethanol, 100% ethanol, and air-dried. The cells were mounted in antifade solution (2% n-propyl gallate in 70% glycerol), and could be viewed immediately by fluorescence microscopy. Image analysis showed that stimulated Jurkat cells were brighter than uninduced controls or those treated with RNase or without polymerase or primer. FIST appears to be useful for the detection of specific mRNAs in single cells.