Streptolysin is a hemolysin produced by group A
streptococci. In an infected individual streptolysin O acts as a protein
antigen, and the patient mounts an antibody response. A rise in
antibody level begins about one week after infection and peaks two to
three weeks later. In the absence of complications or reinfection, the
ASO titer will usually fall to preinfection levels within 6 to 12
months. Both clinical and laboratory findings should be correlated in
reaching a diagnosis.