Recent reports have suggested that mast cells are capable of producing and releasing a number of pro-inflammatory cytokines. However, these studies have mainly been carried out using murine tissue culture derived mast cells and it is known that these cells differ markedly in their functional properties from isolated human mast cells. It was therefore essential to study the release of cytokines from the latter cell type. On immunological stimulation with anti-immunoglobulin E (anti-IgE) or stem cell factor (SCF), purified human lung mast cells released, within 2-10 min, small amounts of tumour necrosis factor-alpha (10.5 +/- 2.9 pg/10(6) mast cells and 17.9 +/- 7.9 pg/10(6) mast cells, respectively) and interleukin-4 (5.3 +/- 2.5 pg/10(6) mast cells and 8.0 +/- 3.2 pg/10(6) mast cells, respectively). After longer periods of activation (30 min-4 h). the amounts of cytokines released from stimulated cells decreased to levels which were below those of the unstimulated cells. This possible degradation of cytokines by mast cells could not be prevented by the addition of protease inhibitors.