Amelogenins are a unique group of alternatively spliced proteins. While the full-length amelogenin is known to assemble into nanospheres and alter apatite crystal growth and alignment, the function of the leucine-rich amelogenin peptide (LRAP) in biomineralization is not understood. This study tested the hypothesis that LRAP self-assembles into a supramolecular structure and guides crystal growth similarly to the full-length protein. Synthetic LRAP and recombinant full-length amelogenin (rH175) were used at different concentrations and either immobilized onto fluoroapatite substrates (FAP) or immersed into saturated calcium-phosphate solutions. The structure of the assembled protein and the height of apatite crystals formed on the FAP template were determined using atomic force microscopy. Both LRAP and rH175 assembled into nanospheres. LRAP self-assembly, however, was only observed at concentrations of > 0.5 mg ml−1 and limited to sizes between 5 and 30 nm. Apatite crystal growth was not significantly affected by LRAP, while rH175 accelerated crystal growth by up to 50-fold. The increased growth rate was only observed when rH175 precipitated at concentrations of > 0.8 mg ml−1. It was concluded that the ability of amelogenins to self-assemble into nanospheres and to bind to apatite in vitro is not inevitably an indication for the ability to control apatite crystal growth.