Group A Streptococcus (GAS) is the most common pathogen identified in cases of recurrent acute tonsillitis. Short (~100 nm) cell-surface fibrils that interact with host tissues have been well characterised in GAS, but recently we observed that GAS can also express polymeric fimbriae extending for >1 μm from the surface (WD Smith, RJ Smith and MA Kehoe, unpublished). Similar surface appendages have been associated with adhesion in Gram-negative organisms such as Salmonella typhimurium and Escherichia coli. In this study, we have investigated the role that a fimbriae-associated protein (FimAP1) may have in GAS adherence to tonsil epithelium. Human tonsil tissue was obtained, with informed consent and ethical approval, from patients undergoing tonsillectomy for recurrent acute tonsillitis (N=4). All specimens were transported to the laboratory within 1h of excision. From each tonsil, 12 primary explants of surface epithelium were incubated in RPMI-1640 media, supplemented with 10% foetal calf serum and 1% non-essential amino acids, at 37^oC with 5% CO₂. Wild-type M1 GAS (n=6) or ΔFimAP1-GAS (n=6), both at a concentration of 10⁷ colony forming units (CFU)/ml and expressing green fluorescent protein (GFP) to allow visualisation, were added to the explants for 4h. Following incubation tonsil tissues were fixed, blocked with 0.5% bovine serum albumin and stained for cytokeratin 14, a tonsil epithelial cell marker (Clark et al. 2000), revealed by red-fluorescent Alexa Fluor 568-labelled secondary antibodies. Human epithelial HEp-2 cells were incubated with the same strains of Streptococcus. Explants were examined by laser scanning confocal microscopy. Examination of growth media from all explant models confirmed GFP fluorescence of both wild type and ΔFimAP1-GAS. On examination of tissue samples it was observed that all tonsil explants from all 4 patients showed adherence of wild type GAS to the surface epithelium after 4h. However, the ΔFimAP1-GAS failed to bind to any tonsil samples. Contrasting results were observed in HEp-2 cells. Both wild-type and ΔFimAP1-GAS adherence to HEp-2 cells was observed. Bacterial adherence and colonization is a common requirement prior to infection. The interaction of GAS, a common cause for recurrent acute tonsillitis, with tonsil surface epithelium is believed to occur via a number of adhesins (Jenkinson & Lamont 1997). The inability of ΔFimAP1-GAS to bind to tonsil tissue suggests an essential role for FimAP1 in Streptococcal/tonsil interaction.