Total cell counts and differential patterns in secretion of 52 mammary quarters of 23 Holstein-Friesian cows were determined at intervals in the early dry and the pre- and postpartum periods. Each sample was cultured to determine infection status of the gland. Twelve quarters remained noninfected, and 10 quarters were infected throughout the test period. Patterns of bacterial isolation from the remaining 30 quarters indicated that transient or intermittent infections had occurred. In secretions of noninfected quarters, geometric mean cell counts increased from 3.6 x 10(5) cells/ml on the last day of lactation to 1.8 x 10(7) cells/ml by the 7th day of the dry period. Total cell concentration remained high through most of the dry period; counts decreased in the prepartum period and reached a level of 2.8 x 10(5)/ml by the 6th day after parturition. Macrophages were the predominant cell type through most of the dry period. The proportion of neutrophils was high initially, decreased as the dry period progressed, and tended to increase as parturition approached, whereas that of lymphocytes increased during involution and then decreased near parturition. In secretions from persistently infected quarters, concentrations of total cells, neutrophils, macrophages, and lymphocytes were higher at all sampling times than in secretions from noninfected quarters. Neutrophils were the predominant cell type in infected secretions at all sampling times. Peak cell concentrations of 8.5 x 10(7)/ml in infected quarters were reached in the immediate prepartum period.