The fibronectin (Fn)-binding ability of microorganisms is considered to be involved in their pathogenicities. Granulicatella adiacens, a member of the oral flora and a causative agent of culture-negative infective endocarditis, showed nearly maximum binding to immobilized Fn at pH 7.2 but greatly reduced binding at a slightly higher pH 7.4 and almost no binding at pH 7.6 in the presence of physiological concentration of NaCl (0.15 M). A similar pH-sensitive Fn-binding property was noted with Escherichia coli and Abiotrophia defectiva, but not with Streptococcus pyogenes nor Staphylococcus aureus. In contrast, bindings to laminin and fibrinogen observed for some of these strains were unaffected by the same pH changes. This fastidious pH-dependency of Fn-binding abilities of some bacteria warns that the pH condition must be seriously considered in the in vitro assay of bacterial adherence to fibronectin. (C) 2003 Elsevier Science B.V All rights reserved.