Etching or size-focusing methods are now widespread for preparation of atomically monodisperse thiolate-protected gold nanoparticles. Size-focusing methods are not widespread, however, in the production of water-soluble gold nanoparticles. Reported here is a new method for size-focusing of large gold nanoparticles utilizing p-mercaptobenzoic acid. We observe preferential formation of three large gold nanoparticles with approximate masses of 23, 51, and 88 kDa. On the basis of the stability of these masses against further etching or growth, they appear to be especially stable sizes. These sizes are not prominent after etching challenges with organosoluble ligands, and the 51 and 88 kDa sizes appear to be novel stable thiolate-protected gold cluster sizes. The overall trend in particle size distribution over time is also unusual, with larger sizes dominating at longer time points.