Influence of fat-replacers on process and age (up to 6 months at 5 degrees C) induced water soluble nitrogen generation in lowfat (5%, w/w) Cheddar cheese was investigated. Treatments with cellulose plus guar gum based Novagel NC200 (NOV), starch based Stellar (STEL), and protein based Dairy Lo (DL) and Simplesse (SIMP) were compared with lowfat (LFC) and fullfat (32%) controls. Nitrogen contents in aqueous extract were determined by Kjeldahl method. Process induced proteolysis followed the order DL>NOV>SIMP> STEL > LFC > FFC. Age related proteolysis, which generated hydrophobic peptides, was rapid in the first 3 months, and resulted in an increase of non-protein nitrogen compared to post-process content by 113, 50, 17 and 14016, respectively for LFC, STEL, FFC, and NOV. Protein based DL showed a reduction in this value by 22% and SIMP showed no change. The ultrastructure of the DL treated cheese matrix appeared closer to FFC compared to all other treatments and the LFC in that smaller (< 3 mu) globular occlusions, representing butter-fat droplets, were seen compared to the dominance of larger (>3 mu) non-globular occlusions in the NOV and SIMP treatments. While the LFC showed signs of globular coalescence and enlargement, DL, and to a lesser extent STEL treatments showed markedly less. The possible reason for the treatment effect on the cheese matrix ultrastructure is discussed.