The properties of human pulmonary mast cells obtained by enzymic dispersion of whole lung and by bronchoalveolar lavage (BAL) have been compared with those of the basophil leucocyte. The latter cell types responded with release of histamine to challenge with anti-human IgE but the dispersed cells reacted only after passive sensitisation with serum from an atopic donor. Disodium cromoglycate inhibited the release of histamine from both types of pulmonary mast cell although the characteristics of the inhibition were different in the two cases. The drug was ineffective against the basophil. Increased numbers of mast cells were recovered by lavage of asthmatic subjects and these cells responded to immunological challenge with an enhanced release of histamine. The possible clinical significance of these findings in human bronchial asthma is discussed.