A negatively-charged Staphylococcus antigen contains amino acids and a N-acetylated hexosamine as a major carbohydrate component. The antigen is common to many coagulase-negative strains of Staphylococcus, including S. epidermidis, S. haemolyticus, and S. hominis. Staphylococcus strains that carry the antigen include many clinically significant strains of Staphylococcus. The antigen and antibodies to the antigen are useful in kits and assays for diagnosing Staphylococcus infection. Vaccines of the antigen and of whole cells that carry the antigen also are disclosed.