A caprine model was used to study Brucella abortus infection of the mammary gland and supramammary lymph node. Peripartum goats were inoculated intravenously, and tissues were examined by light and electron microscopy and bacterial isolation procedures. Brucellar antigen was labeled in situ with immunohistochemical and immunogold techniques. In mammary glands, there was a lymphoplasmacytic and histiocytic, lobular interstitial and periductal mastitis. Brucellae were first seen in mammary glands at 7-15 days postinoculation in macrophages and neutrophils in alveolar and ductal lumens, between alveolar epithelial cells, and in intralobular septae. Brucellae progressively increased in numbers in chronically infected mammae and localized in phagosomes and phagolysosomes in phagocytic leukocytes, which were located primarily in ducts and alveoli, and less often in periductal interstitium, intralobular septae, lymphatics, and blood vessels. Extracellular brucellae were associated with degenerate and disrupted infected phagocytes. Supramammary lymph nodes were enlarged and hyperplastic. Rarely, brucellae were seen in macrophages throughout lymph nodes draining infected mammae, but were most frequent in subcapsular sinuses. This study suggests that macrophages and neutrophils: (i) transport B abortus from the blood into mammary glands and milk, (ii) provide a site for intracellular replication in mammary glands, and (iii) transport brucellae from mammary glands to supramammary lymph nodes in lymph;In a second study, the effect of nursing by peripartum goats on B abortus infection of mammary glands was evaluated. Goats inoculated intravenously at parturition were killed 28 days later. Four goats nursed their neonates and four did not. Brucellae were continuously excreted at high concentrations in milk, and were isolated from udders of 4/4 nonnursing goats. In contrast, brucellae were intermittently excreted in milk in low concentrations and were isolated from udders of 1/4 nursing goats. Brucella-induced lesions were seen in mammary glands from nonnursing goats only. These findings suggest that nursing interferes with B abortus localization and replication in ruminant mammary glands.