Attempts to promote crystal growth in maturation stage enamel from rat incisors were carried out by incubation in saturated solutions of calcium phosphate. Resulting crystallites were visualised in the TEM and the dimensions of their profiles measured. No crystal growth was observed unless the maturation stage enamel was first pretreated with either 8M urea or sodium hypochlorite to remove residual protein matrix. The results suggest that the protein matrix plays an important role in the control of crystal growth in vivo.