In three consecutive experiments. frames without heads from Atlantic salmon were hydrolysed by Protamex(TM) protease. In experiment I (2(4) factorial design) nitrogen recoveries (NR) of 44-76% were obtained, after 120 min of hydrolysis, by systematically varying the hydrolysing parameters, starting pH (5.4-7.7). temperature (36-50 degreesC) E:S ratio (30-90 AU kg(-1) crude protein) and frames-water ratio (0.71-1.21) between low and high levels, The highest levels of E:S ratio (90 AU kg(-1) crude protein), temperature (50 degreesC) and starting pH (7.7) and the lowest level of frames-water ratio (0,71) resulted in the highest NR (76%). In experiment II, the optimal temperature and starting pH in the hydrolysis with Protamex(TM) were established at 50-56 degreesC and 6.5-7.6, respectively. Beyond these temperature and starting pH intervals, the NR tended to decrease. In experiment III, at optimal temperature (50 degreesC) and starting pH (6.5) conditions and frames-water ratio of 1.2, NR of 43-61% were achieved after 60 min of hydrolysis, by varying the E:S ratios from 10-90 AU kg(-1) crude protein. The highest E:S ratio resulted in the highest NR but running the hydrolysis for a period longer than 60 min resulted in little increase in NR. (C) 2002 Elsevier Science Ltd, All rights reserved.