The characteristics of the T-cell population in the healthy human lung have been investigated by analysing the properties of T-cell clones derived from bronchoalveolar lavage (BAL) samples and comparing them with T cells cloned from the blood of the same individuals. The proportions of CD4+ and CD8+ T cells in the starting populations from BAL and blood were similar although only 14% of BAL T cells were CD45RA+ compared to 70% of blood T cells. The precursor frequency of T-cell clones derived from BAL was less than from blood. The cytokine profiles [after phytohaemagglutinin (PHA) stimulation] of the clones derived from both sources were markedly different and these differences lay in the CD4+ population. BAL-derived CD4+ clones produced interferon-gamma (IFN-gamma) more frequently than did those from blood while blood-derived clones were more likely to produce interleukin-2 (IL-2) than those from BAL. IL-4 was produced by the majority of BAL- or blood-derived clones (93% and 88% respectively) either along with IFN-gamma (BAL) or IL-2 (blood). The cytokine profiles of BAL-derived T-cell clones are consistent with those derived from lung interstitium and suggest that the BAL T-cell populations reflect those in the lung wall. Whether the unique properties of lung T cells are acquired after leaving the blood or whether there is selective entry of T-cell subpopulations into the lung remains to be determined.