Hydrolysis of the major caseins in Serra cheese manufactured from raw sheep's milk coagulated with a plant rennet ( Cynara cardunculus, L.) was monitored by urea-PAGE electrophoresis throughout a 35 day ripening period (with sampling at 0, 7, 21 and 35 days) and throughout the cheesemaking season (with sampling at November, February and May). The α s - and β-caseins were degraded up to 82 and 76%, respectively, by 35 days of ripening. The α s -casein variants ( α s2 - and α s3 -) displayed similar degradation patterns to one another, but different from those of β-casein variants ( β 1- and β 2-). Although the α s -caseins were broken down more slowly than β-caseins at early stages of ripening (97, 95, 80, and 60% of α s2 -, α s3 -, β 1-, and β 2-caseins, respectively, were still intact by 7 days), this observation was reversed for later stages of ripening (18, 18, 30, and 20% of α s2 -, α s3 -, β 1− and β 2-caseins, respectively, were still intact by 35 days of ripening). The position along the cheese-making season significantly affected the hydrolysis of only the β 2- and α s3 -caseins. Degradation of α s3 -casein was slower in February than in November or May for 21-day old cheeses; cheeses ripened for 7 days or 21 days showed more intact β 2-casein when manufactured in May than in November or February. The magnitude of the correlation coefficients pertaining to concentrations of intact α s - and β-caseins indicated that the products of proteolytic breakdown with higher mobility than α s -caseins (tentatively termed α 1- I, α 2- I, and α 3- I) were preferentially correlated with α s -caseins, the products of proteolytic breakdown with mobility between β-caseins and α s -caseins (tentatively termed β 1 and β 1- I) were preferentially correlated with β 1 and β 2-caseins rather than with α s -caseins, and the products of proteolytic breakdown with the highest mobility (tentatively termed α β 1 -II and α β 2 -II ) were preferentially correlated with β-caseins.