Individual milk samples were obtained monthly from November 1979 to November 1981 from approximately 2,800 Holstein cows in 63 herds enrolled in the Quebec Dairy Herd Analysis Service. These milk samples were analyzed for fat, total protein, casein, and serum protein. After editing, the data comprised 2,813 lactations representing 109 sires. By approximate maximum likelihood procedures, the model included herd (absorbed), year-month, and parity fixed effects, and sire and error random effects. Lactation casein yield increased with advancing parity to parity five and then decreased slightly. Lactation casein percent and number (percent casein in protein) declined until parities four and three, respectively, and essentially remained constant thereafter. Lactation casein yield was high for lactations initiated during winter months as opposed to summer months. No seasonal trend was apparent for casein percent or number. Heritabilities for lactation casein yield, percent, and number were .11, .26, and .08. Genetic and phenotypic correlations of casein yield with milk, fat, and protein yields were large and positive. Genetic correlation of casein percent with milk yield was negative (-.76) but positive (.96) with protein percent. Genetic correlations involving casein number tended to be small.