Examination of sectioned, embedded material showed that almost all intermediate filaments normally present in cultured chick embryo fibroblasts, IMR-33 cells, and BHK-21 cells were eliminated by trypsinization. At the same time atypical filaments of intermediate size were formed by close apposition of thin filaments. Previous observations (Buckley et al., 1978, J. Cell Biol. 78:644-652) on negatively stained and on critical-point-dried whole-cell mounts confused these different filament types and, on the basis of heavy meromyosin binding to the atypical intermediate-sized filaments, we suggested that many normal intermediate filaments contain f-actin. The present results show that this suggestion was unwarranted and we therefore retract it.

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