Bronchoalveolar lavage fluid (BALF) and peripheral blood were obtained from each of 17 adult Arabian mares and absolute numbers and relative lymphocyte proportions were determined for total T lymphocytes, using CD2 as a marker, CD4 + T lymphocytes, CD8 + T lymphocytes, CD5 + lymphocytes, and sIgM + B lymphocytes. The marked variation in BALF cell recovery resulted in wide variation in absolute values for each lymphocyte subset. The relative proportions of gated BALF lymphocytes were much less variable and provided a basis for comparison of lymphocyte subsets between the BALF and peripheral blood in normal horses. The total lymphocyte population was comprised predominantly of T cells in both BALF (83 +/- 11% CD2 + lymphocytes) and peripheral blood (70 +/- 8% CD2 + lymphocytes). There was a significantly greater percentage of total T lymphocytes and a significantly lower percentage of B lymphocytes in BALF as compared with peripheral blood collected at the same time. The increase in total T lymphocytes in BALF reflected a significantly higher percentage of CD8 + T lymphocytes in BALF (39 +/- 7%) as compared with peripheral blood (18 +/- 5%). The mean percentages of CD4 + T lymphocytes (58 +/- 13% in BALF; 62 +/- 8% in peripheral blood) and CD5 + lymphocytes (78 +/- 11% in BALF; 71 +/- 7% in peripheral blood), a population composed primarily of T lymphocytes but also including a subset of sIgM + B lymphocytes, were not significantly different between the two compartments. These normal values in Arabian mares provide a basis for detection of disease associated changes in the lymphocyte populations and for determination of effects of age and breed on lymphocyte subpopulations in the BALF and peripheral blood.