In 1989, Dameron et al. reported that two yeasts, Schizosaccharomyces pombe and Candida glabrata, produced cadmium sulfide in the form of peptide-capped crystallites as a result of being cultured in the presence of cadmium salts. These can function as quantum semiconductor crystallites. We have now found that the intracellular cadmium sulfide (CdS) quantum semiconductor crystallites approximately 1.8 nm in diameter can be selectively released from S. pombe cells by the effects of freeze-thaw. This method of production does not require cells to be broken as do previously reported methods for CdS crystallite preparation and so the starting material, for subsequent downstream processing, is significantly cleaner and much simpler to purify. We also have exploited the effects of "aging" partially purified CdS quantum semiconductor crystallites of 4 degrees C to minimize the extent of protein cross-contamination and select for the most stable CdS quantum semiconductors. Breakdown of imperfectly coated CdS crystallites evidently removed these. S. pombe was used in preference to C. glabrata for the production of CdS quantum semiconductor crystallites for reasons of microbiological safety. S. pombe also produces a size range of CdS quantum semiconductor crystallites which can be separated by gel filtration to provide quantum semiconductor crystallites with different, highly specific electrooptical properties.