Abstract: A method of determining the gram sign of a bacterial sample is disclosed. In an embodiment of the method, the bacterial sample is first immobilized on a solid support. After immobilization, the bacterial sample is contacted to a peptidoglycan binding ligand (e.g., a ligand which selectively binds to N-acetylglucosamine). The ligand has a detectable label bound thereto. A preferred ligand is wheat germ agglutinin, and a preferred label is fluorescein. After being contacted to the ligand, the bacterial sample is washed with a wash liquid, the wash liquid being provided for a time effective to substantially remove the lectin from gram negative bacteria and ineffective to substantially remove the lectin from gram positive bacteria. The presence or absence of label bound to the bacterial sample is then detected. The presence of label bound to the sample indicates that bacteria in the sample are gram positive bacteria.