Expansion of the lysozyme gene family is associated with the evolution of the ruminant lifestyle in ruminant artiodactyls such as the cow. Gene duplications allowed recombination between stomach lysozyme genes that may have assisted in the evolution of an enzyme adapted to survive and function in the stomach environment. Despite amplification of lysozyme genes, cow tears, milk, and blood are considered to be lysozyme deficient. Here we have identified 2 new cow lysozyme cDNA sequences and show that at least 4 different lysozymes are expressed in cows in nonstomach tissues and probably function as antibacterial defence enzymes. These 4 lysozyme genes are in addition to the 4 digestive lysozyme genes expressed in the stomach, yielding a number of expressed lysozyme genes in the cow larger than that found in most nonlysozyme-deficient mammals. In contrast to expectations, evidence for recombination between stomach and nonstomach lysozyme genes was found. Recombination, through concerted evolution, may have allowed some lysozymes to acquire the ability to survive in occasional acidic environments.