We have prepared murine hybridomas secreting monoclonal antibodies against the exopopolysaccharide xanthan from Xanthomonas campestris pv. campestris 646 after fusing NSO myeloma cells and spleen cells from BALB/c mice immunized with xanthan. Four hybridomas, secreting antibodies designated A6 (IgM kappa), B3 (IgM kappa), D1 (IgM kappa), and D3 (IgG2A kappa), were selected for further studies. All antibodies reacted with a range of different xanthans. Competition studies using variants of the exopopolysaccharide as competitors suggested that specificity was mainly against the side-chain. One of the antibodies (B3) appeared to require the fully acylated side-chain with the pyruvylated terminal mannose as the immunodominant part. The three others were assumed to be directed against the nonsubstituted trisaccharide with the inner mannose-glucuronic acid being immunodominant. None of the antibodies reacted with cellulose (the xanthan backbone). Using immunoblotting techniques on nitrocellulose paper both a mixture of monoclonal antibodies, and also polyclonal ascitic fluid, could detect xanthan quantities of approximately 0.1 microgram.