Danish Blue cheese is a semi-soft blue veined cheese, made from cow’s milk. In addition to proteolytic enzymes, present during normal cheese ripening, the mold Penicillium roqueforti produces aspartyl and metalloproteases that cause considerable changes leading to the unique aroma, flavors and texture of Blue cheese. A study was carried out to investigate the compositional and proteolytic changes occurring in this cheese during 28 weeks of ripening. Moisture levels generally decreased during ripening with concomitant increases in NaCl (~45 to 42% and 3.0 to 3.5%, respectively). Levels of pH 4.6 - soluble N as a percentage of total N increased from 4.2% to 46%, indicating extensive proteolysis during ripening. Urea-PAGE was performed. Before 23 d of ripening, patterns of proteolysis could be explained through the action of chymosin from the coagulant and plasmin from the milk. The action of enzymes from P. roqueforti was apparent in samples ripened for longer periods up to 28 weeks. pH 4.6-Soluble fractions were analyzed by ultra-performance liquid chromatography and showed complex peptide profiles, particularly after 2 weeks of ripening. Extensive proteolysis was associated with the action of the fungal proteolytic enzymes in the cheese. Free amino acid profiling showed an increase in content as ripening proceeded. In an attempt to identify peptides in the cheese produced by mold enzymes, a commercial strain of P. roqueforti PRG-3 was cultured in Potato Dextrose Broth for 7 d at 25°C. Cell-free supernatants were obtained from the culture medium and the action of enzymes on α_S1- and β-casein was determined with resulting peptides identified by ultra-performance liquid chromatography and mass spectrometry. Several peptides found in cheese were thus proven to be produced by the action of fungal enzymes. The results of this study show the extensive proteolysis in Blue cheese later in ripening is mediated mainly by the action of P. roqueforti enzymes.