Acidic proteins are key components of the organic matrix of many biologically formed minerals and are therefore thought to play an important role in their formation. Here we study the effect of one unusually acidic protein of the Asprich family, associated with mollusk shell prismatic layer, on the precipitation of CaCO₃ in vitro. We show that Asprich induces and transiently stabilizes the deposition of amorphous calcium carbonate (ACC). Asprich also induces the formation of ACC when adsorbed onto chitin, a major component of the intracrystalline organic matrix of the prismatic layer. Based on this evidence, combined with previous studies on the forming prisms in the shell layer, we suggest that the in vivo function of Asprich is inducing and stabilizing ACC particles and inhibiting their uncontrolled crystallization until they undergo secondary nucleation on the growing prisms.