Two new proteins, named perlucin and perlustrin, with M-r 17,000 and 13,000, respectively, were isolated from the shell of the mollusc Halotis laevigata (abalone) by ion-exchange chromatography and reversed-phase HPLC after demineralization of the shell in 10% acetic acid. The sequence of the first 32 amino acids of perlucin indicated that this protein belonged to a heterogeneous group of proteins consisting of a single C-type lectin domain. Perlucin increased the precipitation of CaCO3 from a saturated solution, indicating that it may promote the nucleation and/or the growth of CaCO3 crystals. With pancreatic stone protein (lithostathine) and the eggshell protein ovocleidin 17, this is the third C-type lectin domain protein isolated from CaCO3 biominerals. This indicates that this type of protein performs an important but at present unrecognized function in biomineralization. Perlustrin was a minor component of the protein mixture and the sequence of the first 33 amino acids indicated a certain similarity to part of the much larger nacre protein lustrin A.