Three mouse monoclonal antibodies (mAbs) (ICL3, ICL4, and ICL5) were produced that specifically recognized the lipopolysaccharide antigen of the newly recognized Shigella dysenteriae serotype-13 strain. All three mAbs reacted with all nine reference isolates of S. dysenteriae 13 in different tests. The mAbs also detected colonies of S. dysenteriae-13 isolates by direct slide agglutination test. The mAbs also reacted with the reference Escherichia coli 0150 strain and showed its close antigenic relationship with S. dysenteriae 13. Use of these mAbs in our clinical laboratory during an 8-month period detected three S. dysenteriae-13 isolates that were also detected by a polyclonal rabbit antiserum. It should now be possible to define the epidemiologic importance of S. dysenteriae serotype 13 in diarrhea by using these mAbs.