Endotoxemia is associated with an early phase of pulmonary hypertension and a later increase in microvascular permeability. These physiologic changes are attended by peripheral blood and lung lymph leukopenia and a rapid accumulation of both granulocytes and lymphocytes in the peripheral lung. In the present study, the numbers of lymphocytes in blood, lung lymph, and lung tissue after infusion of endotoxin were determined by fluorescent labeling of lymphocyte populations with monoclonal antibodies to sheep T1, T4, T8, or leukocyte common antigen and with rabbit anti-sheep immunoglobulins (B cells). Peripheral blood, lung lymph, and lung tissue samples were collected at baseline and 15, 30, 60, 120, 180, and 240 minutes after the start of intravenous infusion of E. coli endotoxin (1.25 micrograms/kg, N = 6) or saline (N = 4) from open-chest anesthetized sheep. Pulmonary artery pressure and lung lymph flow were also monitored at these times. Endotoxin caused marked reductions in the number of T and B lymphocytes in blood and lung lymph. As compared with baseline, total blood leukocytes and granulocytes were significantly reduced below control levels from 30 minutes of endotoxin, and lymphocyte numbers were reduced from 60 minutes. T1, T4, T8 and B lymphocyte subsets contributed to the fall in blood lymphocytes. Endotoxin caused a significant fall in number of lung lymph leukocytes (T1, T4 and T8 cells) from 120 minutes; numbers of B lymphocytes were also reduced. Counts of the number of lymphocytes in the biopsy tissue revealed a significant rise in T lymphocytes sequestered in the lung. We conclude endotoxemia in sheep causes a reduction in lymphocytes in blood and lung lymph and an increase in these cell types in peripheral lung tissue. These findings suggest that lymphocyte subpopulations accumulate in the lungs during periods of pulmonary hypertension and increased permeability and may participate in endotoxin-induced lung injury.