Crocidolite asbestos is a known human carcinogen containing 27% iron by weight. It has previously been shown that iron was mobilized intracellularly from crocidolite after treatment of human lung epithelial cells (A549) and that the toxicity of the fibers was directly related to how much mobilized iron was in the < 10,000 MW (low-molecular-weight, LMW) fraction [C. C. Chao, L.G. Lund, K. R. Zinn, and A. E. Aust (1994) Arch. Biochem. Biophys. 314, 384-391]. The data here show that iron mobilization from crocidolite began immediately after treatment of the A549 cells and increased linearly with time. However, the synthesis of ferritin, an iron storage protein, did not begin until after 4 h of treatment, reaching a sustained maximum after 12 h. Mobilized iron was preferentially incorporated into the nonferritin-protein fraction up to 7 h after treatment, when the amount of iron mobilized was low and before significant accumulation of newly synthesized ferritin had occurred. This suggested that these cultured cells needed additional iron for synthesis of iron-requiring proteins and that iron mobilized from crocidolite could be utilized directly for this purpose. Subsequent to this, additional mobilized iron was incorporated into newly synthesized ferritin. Even though iron from crocidolite was incorporated into newly synthesized ferritin or into other proteins, the amount of iron from crocidolite in the LMW fraction remained constant during the 24 h. Thus, it appeared that synthesis of ferritin may not have fully protected the cells from the toxic effects of iron mobilized from crocidolite.