The large respiratory epithelial cells within primary cultures of newt (Taricha granulosa) lung are uniquely suited for high resolution video-enhanced light-microscopic studies. We show here that these cells incorporate crocidolite asbestos fibers within 18 h by endocytosis. Once inside the cell, fibers less than 5 microns in length are seen by video light microscopy to undergo saltatory transport at a maximum velocity of 1.18 microns/s. By contrast, fibers over 5 microns long rarely exhibit saltatory motion. Over time, all of the fibers become preferentially located near the nucleus. This perinuclear accumulation is largely inhibited by disassembling the cytoplasmic microtubules with nocodazole. Same cell correlative light and electron microscopy reveal that fibers exhibiting saltatory behavior are enclosed within a membrane. From these observations we conclude that, upon incorporation into epithelial cells, asbestos fibers undergo size-dependent active transport along cytoplasmic microtubules. Our data are the first to link the dimension-dependent transforming ability of asbestos fibers to a basic cellular function, i.e., the microtubule-dependent transport of cellular components.