Lysozyme, a major component of egg white proteins, has been speculated to participate in the calcification of avian eggshells. However, its detailed role during the eggshell formation is not well understood. In this work, the influence of lysozyme on the precipitation of CaCO(3) has been investigated using a combined study of FTIR, XRD, and SEM. The precipitation was produced from (NH(4))(2)CO(3) vapor diffusion into CaCl(2) aqueous solution using a specially built chamber. In the absence of lysozyme, hexagonal platelets of vaterite and their spherical aggregates dominated the precipitates during the first 3-12 h crystallization period studied, with the (001) crystal face well expressed in the hexagonal direction. In contrast, calcite was favored to precipitate in the presence of lysozyme during the same period and the effect was found to be proportional to lysozyme concentration. Furthermore, the (110) face of calcite was expressed in addition to the common (104) face, and the morphological modification was also lysozyme concentration dependent. We attributed these phenomena to the selective adsorption of ammonium ions and lysozyme onto different crystal faces. Our findings have clearly revealed the concentration and face dependent role of lysozyme in CaCO(3) precipitation. This, together with the abundance of lysozyme in the uterine fluid, implies its direct contribution to the hierarchical structures of calcite during the initial stage of eggshell formation.