A surface antigen consisting of aminoglucuronic acid and N-acetyl-L-alanine was isolated from the culture filtrates of two human strains of Staphylococcus aureus. Double diffusion analysis in agar suggested that the antigen is immunologically similar to the alanyl-aminoglucuronic acid capsule of the Smith strain of S. aureus. Quantitative precipitin inhibition studies indicated that N-acetyl-L-alanine is the immunodominant determinant of the acidic antigen. In addition, conjugates consisting of N-acetyl-L-alanine coupled to bovine serum albumin gave a significant precipitin reaction with anti-staphylococcal serum which is rich in alanyl-aminoglucuronic acid polymer antibodies. Antibodies with N-acetyl-L-alanine specificity were isolated from N-acetyl-L-alanine-Sepharose immunoabsorbent columns. Double diffusion analysis in agar indicated that the eluted antibodies were serologically reactive and belonged to the IgG class of immunoglobulins.