The antibacterial activity of purified rabbit lysozyme was kinetically investigated at concentrations comparable to those in normal rabbit serum and plasma serum. The bactericidal capability, lysozyme content, and electrophoretic composition of “purified β-lysin,” fractionated from normal rabbit serum, were also examined. In contrast to the extensive antibacterial activity of dilute normal rabbit serum observed in vitro, rabbit lysozyme was only weakly bactericidal for Bacillus subtilis. Inhibition of lysozyme enzymatic and bactericidal activities in normal rabbit serum by antilysozyme immunoglobulin G slightly reduced the initial rate of killing. The addition of neutralizing antibody or histamine (another lysozyme inhibitor) to partially purified bactericidal serum fractions had no effect on killing kinetics. Increasing the ionic strength of reaction mixtures containing normal serum or partially purified bactericidal fractions to levels which completely inhibited lysozyme activity resulted in stimulation of their respective killing kinetics. The addition of inhibitors to normal rabbit plasma serum completely eliminated its bactericidal activity. With regard to the killing of B. subtilis by rabbit and human blood fractions, these analyses clearly demonstrated that (i) although lysozyme is not a significant antibacterial component of normal rabbit serum, it represents the principal factor in normal rabbit plasma serum; (ii) different primary bactericidal mechanisms which are not detectable by singlepoint analyses operate in the sera of different species; and (iii) purified β-lysin isolated from normal rabbit serum by the classical procedure is a heterogenous mixture of components.
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