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'USG, Chicago, Illinois; 2Schuller; 3OCF, USA; 4Rockwool, Denmark; 5RCC, Swiss Biopersistence of commercial man-made vitreous fibers (MMVF) and crocidolite were studied in Fischer 344 rats. MMVF used were size-selected to be rat-respirable, and rats were exposed nose-only 6 h/day for 5 days to gravimetric concentrations (30 mg/m3) of two fiber glass compositionsa rockwool, and a slagwool-or to 10 mg/i3 of long-fibered crocidolite, or to filtered air. Animals were sacrificed at 1 hr, 1,5, 31, 90, 180, 270, 365, and 545 days after exposure stopped. Fibers were recovered from digested lung tissue to determine changes in concentrations (fibers/mg dry lung) and fiber retentions (expressed as percent of day 1 retention [PR]) for selected dimension categories. One-day average concentrations of lungretained MMVF and crocidolite fibers, of diameter .0.5 pm or >20 pm in length, were nearly equal, permitting direct comparisons between MMVF and crocidolite. At 270 days average PR for MMVF .0.5 pm in diameter were from 3 to 6 ± 2% and 27 ± 9% for crocidolite. For fibers >20 pm, PR were 1 to 4 ± 4% for MMVF and 37 ± 20% for crocidolite. At 545 days, MMVF >20 pm in length were at background level while concentration of crocidolite fibers >20 pm in length remained at 2000 ± 400 f/mg DL (dry lung), or 38 ± 9% of day-1 retention. These results suggest strongly that MMVF dissolved or fractured in vivo whereas crocidolite fibers did not change. -Environ Health Perspect 102(Suppl 5): 139-143 (1994)