Streptolysin is a streptococcal hemolytic exotoxin.
Types include streptolysin O (SLO), which is oxygen-labile, and streptolysin S (SLS), which is oxygen-stable.
An antibody, antistreptolysin O, can be detected in an antistreptolysin O titre.
Streptolysin O is hemolytically active only in a reversibly reduced state, unlike streptolysin S, which is stable in the presence of oxygen. Another difference is that SLO is antigenic, while SLS is not antigenic due to its small size.
References
Streptolysin Wikipedia(Text) CC BY-SA