Between November 1979 and November 1981, 41,783 test-day observations were obtained from 63 Holstein herds in the province of Quebec. Measured were milk yield, percentages of fat, protein, casein, and serum protein, and somatic cell counts that had unadjusted means with standard errors 20.44 +/- .04 kg, 3.684 +/- .003%, 3.314 +/- 002%, 2.694 +/- .001, .669 +/- .001%, and 313625 +/- 3238/ml of milk. Casein as a percentage of total protein (casein number) was 79.35 +/- .015. Least squares analyses showed significant effects of herd, calendar month of test, age of sample at laboratory testing, stage of lactation, age of cow, and somatic cell counts on milk yield and composition. Milk yield was higher whereas percentages of fat and protein and somatic cell counts were lower in summer than in winter months. Somatic cells, fat, protein, and casein contents were high during early stages of lactation, reached a minimum at 2 mo in lactation, and rose gradually throughout the rest of the lactation. A comparison of younger cows (less than or equal to 2 yrs) with older one (greater than or equal to 6 yr) revealed that somatic cell counts increased from 166,000 to 507,000/ml of milk and casein number decreased from 80.14 to 78.88. Increase of somatic cell counts in milk was associated with increased protein content, which was mainly from the serum protein fraction as the casein fraction was not related with somatic cells.