The first effect of receptor activation on the mast cell surface, initiating histamine secretion, is an increase in the cytosol Ca2+ concentration. It should then be possible to induce histamine secretion by calcium alone, if the calcium permeability of the cell membrane could be increased without any significant interference with the physiological cell functions. This was achieved in the present study by adding low concentrations of saponin (0.0005% and 0.001% w/v) to the medium. When calcium was added to the saponin-permeabilized cells, around 40% histamine release occurred with 0.25 mM extracellular calcium (free Ca2+ 0.15 mM). The release was inhibited by antimycin A (1 microM). Transmission electron microscopy showed formation of vacuoles containing granules stripped of their membranes, which characterize a secretory response. The observations are consistent with a limited increase in the calcium permeability of the cell membrane for a brief period. There was apparently an increase in the cytoplasmic calcium concentration, which acted through calmodulin, since the histamine release induced by calcium from the permeabilized mast cells could be inhibited by a calmodulin-antagonist, mepacrine (10-30 microM).