A strong lytic activity against Micrococcus luteus was demonstrated in abomasal secretions from calf, adult cattle, goat and sheep. This bacteriolytic activity was undetectable in other secretions. Bacteriolysis was caused by a glycosidase displaying endo-N-acetylmuramoylhydrolase specificity (EC 3.2.1.17) and was further characterized in the calf. This lysozyme also displayed significant chitinase activity. Immunofluorescence microscopy confirmed the secretion of lysozyme by abomasal gastric glands exclusively. Electrofocusing revealed multiple molecular forms, the predominant one (more than 80%) being characterized by Mr approx. 15,000, pH optimum 5.0, pl 7.5 and remarkable conformational stability. The lytic activity of lysozyme was ionic strength dependent and competitive inhibition was observed with both N-acetyl glucosamine and N-acetyl-muramic acid. Amino-acid analysis demonstrated common characteristics with known lysozymes, i.e. four disulphide bridges, two proline and N-terminal lysine. Structural homology between the three ruminant lysozymes was established by immunological cross-reactivity.