Further studies are presented on the intracellular localization of the amyloid-related serum protein SAA previously shown to be produced by embryonal fibroblasts. In cultured embryonal fibroblasts, the fine fibrillar cytoplasmic immunofluorescence obtained by anti-SAA was distinguished from that of microfilaments and microtubules. By using electron microscopy and cells treated with drugs known to specifically alter intracellular fibrils, SAA was localized to 10-nm intermediate size filaments. These filaments form characteristic perinuclear bundles upon treatment with drugs such as demecolcine or vinblastine which disrupt micotubules. The results indicate that SAA is a constituent of the intracellular cytoskeleton.