The population of lymphocytes obtained by endobronchial lavage is significantly different from the distribution of lymphocytes in the peripheral blood. Both B and T lymphocytes are found in fluids lavaged from the normal human lung. Of the lymphocytes that can be classified, the T cell population is larger than the B cell population in both nonsmokers and cigarette smokers, whereas a further increment in T cells is noted in smokers. A third population of unclassified lymphocytes, the "null" cells, also comprise a large portion of the lymphocyte population in both smokers and nonsmokers. Their significance has yet to be determined.