Isolated human lung mast cells were used to identify subcellular sites of basic fibroblast growth factor using a postembedding immunogold method. The factor was present in quantity in secretory granules and cytoplasmic lipid bodies. Cisterns of smooth endoplasmic reticulum and ribosome clusters, closely associated with lipid bodies, contained the factor as did the nuclear matrix. Factor-positive lipid bodies were adjacent to nuclear pores and often indented perinuclear cisternae. Altered secretory granules with reduced density, characteristic of secretion by piecemeal degranulation in mast cells, showed reduced gold label for basic fibroblast growth factor; small, electron-lucent (80-100 nm) transport vesicles near altered granules were labelled for the factor. Since these mature mast cells do not display extensive arrays of classical secretory organelles, such as rough endoplasmic reticulum and Golgi structures, these new subcellular localizations for basic fibroblast growth factor suggest several possible alternative release routes for a cytokine devoid of a signal sequence characteristic of regulated secretory proteins.