Casein micelles of different composition were synthesized in various ways and their sub-structure was investigated with the electron microscope by means of thin sections. Earlier studies of Schmidt et al. (1974) using the freeze-fracturing technique had shown no differences in the sub-structure of natural micelles and artificial micelles containing Ca and casein only. Contrary to these results our present studies showed that for the production of synthetic casein micelles with the same sub-structure as the natural ones it is necessary to add at least 2 other ions to the casein solution besides Ca2+: these are phosphate and citrate. The citrate ions play an important role in forming the material of the dark framework of the micelles visible in electron micrographs of unstained thin sections. This supports the hypothesis of Pyne & McGann (1960) that casein micelles contain a citrate apatite.