In patients with acute bacterial infections antibodies directed against a particular bacterial antigen were detected. The molecular mass of this bacterial antigen was 50 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. By comparison of the NH2-terminal amino acid sequence, the 50-kDa antigen was identified as alkaline phosphatase (AP). Affinity-purified antibodies from patient's sera directed against the bacterial AP (anti-alpha) were also shown to react with human and animal AP, which have different structures. Anti-alpha are IgG subtype 3 immunoglobulins, and their light chains are of the kappa type. Upon isoelectric focussing, the anti-alpha formed a scalariform pattern with five to seven bands in the pH range 7-9. The anti-alpha have an opsonic activity and cause a five- to eightfold increase of phagocytosis of gram-positive and gram-negative bacteria. According to their polyreactivity, their sudden rise early in infection, their oligoclonality, as well as their opsonizing properties, they are assumed to be permanently available natural antibodies that take part in early defence mechanisms.