The existence and distribution of T lymphocyte subpopulations in the mammary parenchymal tissue of cows were immunohistochemically detailed during early lactation. CD2+, CD4+, and CD8+ T lymphocytes were localized primarily in the mammary parenchymal tissue. CD8+ T lymphocytes were predominant over CD4+ T lymphocytes and occurred in close contact with the alveolar epithelium and between epithelial cells in the central area of the upper mammary gland. CD4+ T lymphocytes were present in equal numbers in the epithelial and connective tissue area. Occasionally, both CD4+ and CD8+ T lymphocytes formed cell clusters in the interalveolar connective tissue. The ratio of CD4+ T lymphocytes to CD8+ T lymphocytes was less than 1.0 and was lower in the epithelial area than in the connective tissue. The distribution of CD4+ and CD8+ T lymphocytes was similar in the parenchymal tissue of the gland cistern. The observation that there is a preferential presence of CD8+ T lymphocytes in the epithelial area of the bovine mammary gland during early lactation might indicate that these cells participate in the maintenance of the integrity of the epithelium.