Process for the extraction and purification of proteins from culture media producing them

In a method for extracting and purifying a cell-bound protein from the supernatant of engineered yeast cells having produced such protein and disrupted in the presence of a non-ionic detergent, a process which comprises, as a first step, adjusting the pH of the supernatant to 6 (.+-.0.1), adding thereto either liquid or solid polyethylene glycol up to clarification of said supernatant, and, as a second step, treating the clarified supernatant either with a bivalent metal cation or, before ultrafiltration or after eventual ultrafiltration, with ammonium sulfate for separating said protein.