Aim: The life-threatening group A streptococcal (strep) infection and its sequelae, including acute rheumatic fever (ARF), re-emerged as a serious health problem. The fleeting arthritis of ARF is considered a form of reactive arthritis. However, no one has confirmed this by investigating its synovial fluid cells for a possible presence of strep cell wall antigens using western blot in humans. This is the aim of the current study. Methods: Synovial fluid- (SF) and peripheral blood-mononuclear cells (PB-MNCs) from 40 patients with ARF and 10 patients with rheumatoid arthritis (RA), who served as a control group, were examined for strep antigens by immunofluorescence (IF) and western blot (WB) techniques using rabbit polyclonal antiserum and mouse monoclonal antibodies. Results: Extensive bacterial cultures of SF, blood and throat were negative. By IF, a significant proportion (37.5%) of ARF samples (Chi-square=3.72, p=0.048) showed positive staining in SF- as well as PB-MNCs with both rabbit polyclonal antiserum and mouse monoclonal antibodies. Further, IF was significantly higher in ARF- than RA-patients (Mann-Whitney p=0.022) in whom we failed to observe any staining. By immunoblotting, 21 samples from ARF patients (52.2%) were positive with mouse monoclonal antibodies specific for strep peptideglycan-polysaccharide (PG-PS) complex in SF-Cs (a band with a molecular weight of 28 kD) and PB-MNCs (29kD) and its proportion was significant (p=0.0008). With rabbit polyclonal antiserum, significant blots (p=0.027) were noted in 27/40 ARF patients (67.5%) indicating strep PG-PS (24-29kD broad band) in SF-Cs and PB-MNCs. Blots by both mono- and poly-clonal antibodies were significantly higher (p=0.003&=0.001, respectively) than control samples that were non-reactive using both types of antibodies. Conclusion: The reactive nature of acute rheumatic fever is suggested by the frequent detection of streptococcal cell wall antigen from affected joints using both, immunofluorescence and western blotting.