Dictionary.net

Substances that are toxic to leukocytes. Specifically exotoxins produced by some pathogenic staphylococci and streptococci that destroy leukocytes by lysis of the cytoplasmic granules and are partially responsible for the pathogenicity of the organisms. (Dorland, 28th ed)

[Latin] A substance occurring in staphylococcus cultures, which paralyses and destroys leucocytes.