Twenty-five quarters of 12 dairy cows, 3 to 8 years old, with a bacteriologic history of freedom from infection with Streptococcus uberis were inoculated via the teat canal with S uberis (23 quarters) or sterile medium (2 quarters). The cows were sent to slaughter 1, 3, or 6 days later. Acute inflammatory response involving accumulation of large numbers of polymorphonuclear, neutrophilic leukocytes (neutrophils) in the secretory acini was recognized after 24 hours in infected cows. After 6 days, the neutrophil response was still evident, but infiltration of septa by lymphocytes, septal edema, extensive vacuolation of secretory cells, focal necrosis of alveoli, small outgrowths of the secretory and ductular epithelium, and widespread hypertrophy of the ductular epithelium also were recognized. Early stages of involution and fibrosis also were evident at that stage. Streptococci were identified by immunoperoxidase labeling, free or phagocytosed, in macrophages; in the alveolar lumina, adherent to damaged secretory or ductular epithelium; in the subepithelium and septal tissue; and in lymphatic vessels and lymph nodes. The importance of the macrophage as the primary phagocytic cell is highlighted, and doubt is cast on the value of the exuberant neutrophil response by the host in defense of the gland.