Alveolar macrophages (AM), together with other cells such as eosinophils, have been implicated in the bronchial inflammation associated with asthma. Examination of bronchoalveolar lavage (BAL) fluid has shown that these cells are activated and that the degree of activation is correlated with both the clinical severity of asthma and with the level of bronchial hyperreactivity. The cell population recovered by BAL is normally heterogeneous with regard to AM and this study was designed to investigate whether the proportion of hypodense AM is greater in asthmatics (n = 12) compared to normal subjects (n = 10). Our results show that AM from asthmatic donors are mainly found in the fractions corresponding to low densities (1.03 to 1.04 g/ml) whilst those from normal donors are found at higher densities (1.07 g/ml). Examination of the cells by electron microscopy showed that the hypodense AM displayed the morphological characteristics of activated cells in contrast to the normodense AM which had the characteristics of quiescent cells. In order to determine whether this increased activation was a result of an expansion in the AM population following recruitment of monocytes from the medulla, we studied the functional characteristics of cells recovered from each of five fractions as assessed by their ability to release oxygen-derived free radicals. No differences were observed between AM recovered from the five different density fractions in either asthmatics or in normal subjects. In conclusion, if there is a difference in the density of AM from asthmatics as compared to normal subjects, it is unliked to be due to the recruitment of new cells, but rather a result of an accelerated progression from a mature but resting state to an hyperactivated state.