Fifty murine monoclonal antibodies were produced against a purified preparation of K88ab antigen. Two monoclonal antibodies were selected for further studies together with a monoclonal antibody directed against the c region of K88 (5CA3). Monoclonal antibody K88-13 bound to all K88+ Escherichia coli examined, whereas K88-18 and 5CA3 bound to K88ab+ and K88ac+ strains, respectively. The monoclonal antibodies failed to react with K88+ E coli grown at 18 degrees C or with K88- E coli grown at 37 degrees C or 18 degrees C. Binding of monoclonal antibody K88-13 to K88 antigen was blocked by OK antisera to G7 (O8:K87 K88ab) and Abbotstown (O149:K91 K88ac), whereas absorbed antisera to K88b, c and d had no effect. Monoclonal K88-18 was inhibited by OK G7 antiserum and absorbed antiserum to K88b. One hundred and forty-nine strains of K88+ E coli representing seven somatic O groups were agglutinated by antibody K88-13; 142 of these, from at least six somatic O groups were agglutinated by 5CA3 and the remainder by K88-18. Monoclonal antibody K88-13 bound to cryostat sections of ileum from piglets infected with E coli strains Abbotstown and G7. K88-18 and 5CA3 bound only to sections from piglets infected with G7 and Abbotstown strains, respectively. It is concluded that monoclonal antibody K88-13 recognises an epitope on the a region of K88 while monoclonal antibody K88-18 is directed towards the b region. These monoclonal antibodies together with 5CA3 can be used to routinely identify K88+ E coli isolates.