One hundred twenty-seven Jersey cows (1 to 6 years of age) within a 160.93-km radius of Baton Rouge, La, were selected at random over a 3-year period to determine serum values for total creatine kinase, aspartate transaminase, calcium, phosphorus, alkaline phosphatase, cholesterol, total protein, globulin, urea nitrogen, and glucose. Breed and age were partitioned as possible sources of variation. Seasonal temperature and management influences were taken into account by restricting the sampling area. Seasonal temperature and management influences along with reproductive status and stage of lactation, were randomized by sampling over the 3-year period. All serum values, except aspartate transaminase, urea nitrogen, and glucose were influenced by age. Serum cholesterol, total protein, and glucose were influenced by age. serum cholesterol, total protein, and globulin tended to increase with age, whereas creatine kinase, calcium, phosphorus, and alkaline phosphatase generally decreased with age.