F ibroblast growth factor (FGF)-1 and PDGF-B-like factors have been implicated in the pathobiology of RA and animal models of this disease. Since the receptors for FGF-1 and PDGF are tyrosine kinases, we examined the expression of tyrosine phosphorylated proteins (phosphotyrosine, P-Tyr) in synovial tissues from patients with RA and osteoarthritis (OA), and rats with streptococcal cell wall (SCW) and adjuvant arthritis (AA). Synovia from patients with RA and LEW/N rats with SCW and AA arthritis, in contrast to controls, stained intensely with anti-P-Tyr antibody. The staining colocalized with PDGF-B and FGF-1 staining. Comparative immunoblot analysis showed markedly enhanced expression of a 45-kD P-Tyr protein in the inflamed synovia. Treatment with physiological concentrations of dexamethasone suppressed both arthritis and P-Tyr expression in AA. P-Tyr was only transiently expressed in athymic nude Lewis rats and was not detected in relatively arthritis-resistant F344/N rats. These data suggest that (a) FGF-1 and PDGF-B-like factors are upregulated and may induce tyrosine phosphorylation of proteins in vivo in inflammatory joint diseases, (b) persistent high level P-Tyr expression is T lymphocyte dependent, correlates with disease severity, and is strain dependent in rats, (c) corticosteroids, in physiological concentrations, downregulate P-Tyr expression in these lesions.