It is of interest to determine whether and how egg-white proteins are maintained in fertile eggs. We previously observed that egg-white ovalbumin attained high stability during embryogenesis. Herein, we observed that the total mass of egg white and that of its gross protein content showed a decrease according to the days of incubation. The total bacteriolytic activity also lowered, in accord with previous observations. We purified lysozyme from egg-white samples on several incubation days. These purified lysozyme proteins were observed to have enzymatic and bacteriolytic activities against Micrococcus lysodeikticus as well as growth-inhibition potency against Staphylococcus aureus. As the embryogenesis proceeded, the purified lysozyme showed changes in Km and Vmax, a small decrease in the denaturation temperature, and symptoms of an increase in surface hydrophobicity. These results indicate that the lysozyme protein maintained its enzymatic and antibacterial activities until the late period of incubation while undergoing slight conformational changes.