Adherence of an encapsulated (UT 101) and a non-encapsulated (UT 102) strain of Streptococcus uberis to a bovine mammary epithelial cell line (MAC-T) and to extracellular matrix proteins (ECMP) including fibronectin, collagen and laminin was investigated. S. uberis was co-cultured at 4 degrees C with MAC-T cell monolayers. Both strains of S. uberis adhered to MAC-T cells. However, the non-encapsulated strain of S. uberis adhered better to MAC-T cells than the encapsulated strain. Preincubation of MAC-T cells with lipoteichoic acid (LTA) and/or treatment of S. uberis with antibodies directed against the carboxyl-terminal half of type 24 M protein reduced adherence of both strains of S. uberis to MAC-T cells. Adherence to ECMP was measured by incubating bis-carboxyethyl-carboxyfluorescein acetomethyl ester (BCECF-AM) labelled S. uberis in 96-well plates coated with fibronectin, collagen or laminin. Both strains adhered to ECMP, however, the encapsulated strain adhered better to ECMP than the non-encapsulated strain. Results of this investigation demonstrated that both strains of S. uberis evaluated were capable of adhering to bovine mammary epithelial cells and to ECMP. Adherence of S. uberis to mammary epithelium may be an extremely important mechanism in the establishment and progression of bovine intramammary infections.