Human alveolar macrophages (AM) were obtained by bronchoalveolar lavage from 18 patients with a variety of conditions. For each patient the percentages of AM showing the following properties were determined: (1) staining for the enzymes non-specific esterase (NSE) and acid phosphatase (ACP); (2) in vitro phagocytosis of Candida guillermondii; (3) expression of cell surface markers detected by two monoclonal antibodies (MoAb) (1B5 and DA2) and two anti-monocyte/macrophage MoAb (UCHMI and RFD2); and (4) simultaneous phagocytosis of C. guillermondii and staining with the MoAb. In all patients the majority of AM were found to be Ia positive (90 +/- 10%) ACP positive (100%) and NSE positive (97 +/- 4%). In contrast a smaller proportion were UCHM1 and RFD2 positive (77 +/- 11%, 68 +/- 12%) and less were phagocytic (37 +/- 17%). Whilst the total percentage of cells staining with the MoAb was unaltered by phagocytosis, the proportion of UCHM1 or RFD2 positive cells was significantly higher in the phagocytic population than in the non-phagocytic population (90% and 85%, as opposed to 65% and 55%, P less than 0.001). Thus only a proportion of Ia positive AM expressed monocyte/macrophage antigens and were phagocytic. Such heterogeneity may reflect different stages of macrophage maturation or the existence of macrophage subpopulations with functionally distinct roles in airways immunity.