Anti-streptolysin O (ASO or ASLO) is the antibody produced against an antigen produced by Lancefield group A streptococci. The enzyme is called streptolysin O, wherein the O stands for oxygen-labile, the other antigen being oxygen stable streptolysin-S. Both the enzymes are involved in producing hemolysis, i.e., digestion of blood, in particular, beta-hemolysis. Synonyms: Antistreptolysin O positive control, ASO reagent.