The ultrastructure of the cytoskeleton of optic nerve axons in guinea pigs was examined by the quick-freezing and deep-etching (QF-DE) method. The optic nerve tissues were treated with 0.5% saponin before QF. In the replicas, the axoplasm, as observed with conventional ultrathin sections, was seen to be composed of longitudinally oriented microtubules (MT) and neurofilaments (NF). Thin and elaborate cross-linking structures were observed in the interstices between MTs, NFs, and membranous organelles. They consisted of two different types: NF-associated cross-linking structures (about 20-50 nm in length) and MT-associated structures (about 10-20 nm in length), and may play a role in the slow transport of NF and MT and fast transport of organelles, respectively.