The hen eggshell is formed in a three-stage sequential process, namely, the initial stage (nucleation of calcite crystals), the active growth phase (linear deposition), and the terminal stage (inhibition of crystal formation). During these phases, different proteins are sequentially expressed in the uterine fluid. Some of them are thought to regulate the shell mineralization and particularly crystal growth. To identify proteins that are actively involved in this process, we have analyzed and compared the effects on CaCO(3) precipitation in vitro of some commercial egg white proteins, some noncommercial purified fractions of the eggshell organic matrix and, also of uterine fluids extracted from the hen uterus during each one of the stages of calcification. Uterine fluids strongly increased the nucleation density with respect to commercial proteins and purified fractions of the shell. A few identified acidic proteins (ovocleidin-17, ovocalyxin-32, osteopontin and ovocleidin-116) showed a strong affinity for the CaCO(3) surfaces and were selectively removed from the solution during its precipitation. These proteins may have an active role on CaCO(3) growth, aggregation, and inhibition. Other proteins, with very different isoelectric points, seem to regulate the chemical environment in which the precipitation takes place, that is, by buffering the pH favoring crystal growth as the couple ovocleidin-17 and ovocalyxin-21.