Cheese whey and cottage cheese whey are by-products of the milk and cheese industry, resulting from the production of cheese and cottage cheese (ricotta) from milk. They are still rich in organic substances and cannot be discarded into the environment without proper treatment. Whey and cottage cheese whey were used as culture media for some strains of the yeast Kluyveromyces lactis, transformed with the human lysozyme gene. It was found that the yeast strains grew well in both media and produced a considerable amount of recombinant protein. Production kinetics showed that the human lysozyme was produced in a greater amount within 36 h of fermentation (125 μg ml−1vs 25 μg ml−1 in the control) than in the synthetic commercial media used for strain preparation and characterization. The recombinant protein produced was actually shown to be the human lysozyme, using renaturing SDS-PAGE and Western blot techniques. While producing recombinant protein, the Kluyveromyces strain cleared the cottage cheese whey of most organic substances and produced a considerable amount (almost 3%) of lysozyme-enriched useful biomass.