Previous studies of the cytokine profile of rheumatoid arthritis (RA) have been primarily limited to the assessment of the levels of these mediators in synovial fluid (SF) or synovial tissues (ST) by biologic or immunologic assays. We have studied cytokine gene expression in RA by in situ hybridization of SF cells, enzymatically dispersed ST cells, and frozen sections of ST. RA ST cells (n = 7) were studied and a high percentage of cells hybridized to the following anti-sense probes: IL-6 = 19 +/- 3.3%; IL-1 beta = 9.9 +/- 1.7%; TNF-alpha = 5.8 +/- 1.4%; granulocyte-macrophage-CSF = 2.2 +/- 0.8%; transforming growth factor-beta 1 = 1.3 +/- 0.2% (p less than 0.05 for each compared to sense probes). Similar results were found using osteoarthritis ST cells, although the percentage of cells expressing the IL-6 gene (7.1 +/- 2.5%) was significantly less in osteoarthritis compared to RA. RA ST cells did not significantly bind the IFN-gamma probe (0.2 +/- 0.1% positive), although they were capable of expressing the IFN-gamma gene if stimulated with PHA. The OKM1+ population of ST cells (i.e., macrophage lineage cells) was greatly enriched for IL-1 beta and TNF-alpha, whereas the OKM1- population (lymphocytes, fibroblasts, and type B synoviocytes) was enriched for IL-6. The vast majority of cells expressing the IL-6 gene were non-T cells. Furthermore, hybridization to RA ST frozen sections localized IL-6 mRNA to the synovial lining layer, which is comprised of type A and type B synoviocytes. In contrast to the high level of cytokine gene expression observed in ST, SF cells did not hybridize significantly to any of the cytokine probes. If stimulated with LPS or PHA, SF cells expressed IL-1 beta or IFN-gamma genes, respectively.