Specificity: Human LYZ / Lysozyme. The reactivity of the antiserum is restricted to lysozyme. In immunoelectrophoresis and radial immunodiffusion (Ouchterlony), using various antiserum concentrations against normal human milk and tears a single precipitin line is obtained which shows a reaction of identity with the precipitin line obtained with purified lysozyme. No reaction is obtained with any other plasma protein component or serum. Cross-reactivity The antiserum does not cross-react with any other component of human plasma. Inter-species cross- reactivity is a normal feature of antibodies to plasma proteins since they frequently share antigenic determinants. Cross-reactivity of this antiserum has not been tested in detail.