Proteolysis and electrophoretic pattern of casein were determined in Friesian cows' skim milk, buffaloes' skim milk and in these milks fermented spontaneously (leben rayeb) and by Streptococcus lactis subsp. diacetylactis DRC3 (nonfat buttermilk) or by zabadi starter (zabadi). The highest proteolysis, as evidenced by the amount of released leucine, occurred in zabadi and the lowest in leben rayeb. The highest amount of tyrosine was liberated in the nonfat buttermilk. Proteolysis seems not to depend on the kind of milk used. A band of a relatively high density appeared to be released from alpha s-casein by the cell-wall proteinase of the microflora of raw milk and by heating at 90 degrees C for 1 min. Cows' skim milk, raw or heated, showed the presence of 2 probable proteose-peptone components; buffaloes' skim milk 3 and 2. These components underwent slow or rapid degradation, depending on the type of fermented milk during skim milk coagulation.