The presence of the indigenous milk alkaline proteinase, plasmin, in whey products may adversely affect the quality of food products that utilise such ingredients; factors promoting the release of plasmin into whey were therefore investigated. Acidification of pasteurised skim milk (PSM) with glucono-delta-lactone (GDL) or HCl resulted in acid whey with significantly higher (P<0.05) plasmin activity than PSM. Plasmin activity in such whey was inversely correlated with rate of acidification; thus, activity in the whey made with GDL was significantly higher (P<0.05) than in whey made with HCl. Negligible levels of plasminogen in acid whey suggested that activation of the zymogen, plasminogen, contributed to the elevated plasmin activity observed. Sweet whey, manufactured by addition of rennet to PSM, had plasmin and plasminogen-derived activities significantly lower (P<0.001) than those in PSM; the release of plasmin from renneted casein micelles into whey increased in a linear manner with cooking temperature in the range 45-65degreesC. However, dissociation of plasmin from casein micelles into acid whey reached a maximum at a cooking temperature of 50degreesC. Analysis of plasmin and plasminogen levels in ultracentrifugal supernatants of pH-adjusted milk indicated that plasmin was released from the micelles at pH > 5.0, while plasminogen was released more gradually over the pH range 6.6-4.6. Overall, the technology of manufacture has significant implications for plasmin activity in whey and hence, inversely, for casein products and cheese. (C) 2003 Elsevier Ltd. All rights reserved.