Nuclear matrices (NM) were prepared from mouse 3T3 fibroblasts attached to growth support film in the continuous presence of the disulfide cross-linking reagent, sodium tetrathionate. Intact cells and samples at each stage of NM preparation were fixed, embedded in Epon-Araldite, sectioned and stained conventionally with uranyl-lead. Nuclear size and shape changed little during extraction, but nuclei showed a gradual reduction in internal fibrogranular elements up to 1M NaCl, after which larger spaces were visible in the nucleoplasm. In contrast to similar samples prepared under reducing conditions in other studies, final NMs contained a highly ramified internal network of fibers and granular aggregates.