The hemolytic activity of a bacterium or molecule is its ability to destroy red blood cells, leading to the release of hemoglobin.
To assess this ability, bacteria are for example grown on blood agar plates. In case of hemolytic activity, the bacteria will form colonies surrounded by a clear zone. The lysis of red blood cells by bacterial toxins makes the medium more or less transparent. This transparency is total if hemolysis is complete. In case of partial hemolysis, a slight haze is observed.
Hemolytic activity can also be assessed by a Red Blood Cell Lysis technique, which consists of bringing the red blood cells into contact with the bacteria or the compound, and quantifying the hemoglobin released by spectrophotometric determination.