Hydrophilic and hydrophobic peptides in the water-soluble fraction of semi-hard, vacuum packed ovine cheeses made with calf, microbial and cardoon coagulants were monitored for 90 d by reversed-phase HPLC. Hydrophobic peptides of the cheeses were further analysed by size exclusion HPLC to determine their molecular sizes and the bitterness of the cheeses was evaluated by a trained sensory panel. Cheese made with cardoon coagulant had the highest levels of both hydrophilic and hydrophobic peptides. Cheeses made with calf and microbial coagulants had comparable levels of hydrophilic peptides, but the former had higher levels of hydrophobic peptides. The concentration of bitter peptides (those with a molecular size of 165-6500 gÂ·mol-1) was highest in cheese made with microbial coagulant and lowest in cheese made with calf rennet. Cheese made with microbial coagulant was perceived to be the most bitter by the sensory panel, followed by calf and cardoon coagulant cheeses. Sensory bitterness score was significantly (P < 0.05) correlated with total bitter peptides and the ratio of bitter peptides to total peptides, but not with total hydrophobic peptides.