In this paper, we report the studies relevant to controlling the size of protein-protected gold nanoclusters (AuNCs). In order to demonstrate this, we have chosen bovine apo α-Lactalbumin (apo-α-LA), which has a specific binding site for Ca2+, and La3+ occupies this position when apo-α-LA is treated with lanthanum trichloride. When the Apo-α-LA is treated with Au3+, it results in the formation of a protein-coated Au10 nanocluster where the protein reduces Au3+ to Au0 and protects the nanoclusters (apo-α-LA-AuNCs) which are luminescent. In these protein-protected luminescent gold nanoclusters, the protein is involved both in reduction as well as protection, thereby supporting green synthesis. As La3+ occupies the Ca2+ site in apo-α-LA, the size of AuNCs formed is reduced to smaller than the Au10, where the size is dependent on the extent to which La3+ is bound to the protein with a concomitant increase in luminescence. The apo-α-LA-AuNCs and the same formed in the presence of different concentrations of La3+-bound protein were all characterized by analytical, spectral, and microscopy techniques. Control of the size of AuNCs formed was also studied by using Gd3+ instead of La3+ and found similar results. In particular, the size variation of AuNCs was clearly demonstrated by MALDI-TOF-MS and HRTEM. Thus, the apo-α-LA protein-coated gold nanocluster is a useful green material for suitable applications. © 2017 American Chemical Society.