Two types of lamb rennet pastes, traditional (A) and industrial (B), were used for making two types of PDO Fiore Sardo cheese (FSA and FSB) to ascertain the influence of their use on cheese characteristics. Traditional rennet was produced from the stomachs of lambs fed only milk and slaughtered, after suckling, on a full stomach. Industrial rennet was produced from the stomachs of lambs weaned and slaughtered on an empty stomach. The cheeses were significantly different (Po0.01) in pH values and pH 4.6-soluble N/total N throughout ripening. FSA cheese had a higher rate of proteolytic activity. Within 240 d of ripening, as1-CN was degraded completely whereas b-CN was partly hydrolysed by plasmin, irrespective of the rennet paste preparation. A number of phosphopeptides were detected in fresh cheeses, which were progressively shortened and dephosphorylated during ripening. In FSA cheese, greater amounts of free fatty acids were released, in particular butyric acid.