Anti-streptolysin o (aso) is the antibody
made against streptolysin o, an immunogenic, oxygen
-labile hemolytic toxin produced by most strains of group a and many strains of groups c and g streptococci.
The o in the name stands for oxygen-labile. The main function of streptolysin o is to cause hemolysis (the breaking open of red blood cells).