The cold non-depolymerizable fractions obtained during the standard procedure for the isolation of microtubules from ox brain stem-cerebral hemispheres and spinal cord have been studied. The cerebral-hemisphere preparation was composed of 10-nm filaments but also contained large amounts of membranes. The polypeptide content included tubulin, microtubule-associated proteins and minor proteins corresponding to the neurofilament triplet of proteins of mol.wt. 210 000, 160 000 and 70 000 respectively. The brain-stem preparation contained more 10-nm filaments than membranes. The polypeptide content consisted of the neurofilament triplet (35%), tubulin (30%) and minor proteins. In contrast, the spinal-cord preparation was mainly composed of 10-nm filaments, free of membranes and containing essentially the neurofilament protein triplet (64%). These filaments appeared very similar to the peripheral-nervous-system neurofilaments described by several authors. Since the best neurofilament from the central nervous system often contained less than 15% of the neurofilament protein triplet, our spinal-cord preparation is an improvement on the usual neurofilament preparation. This simple and rapid method gave large amounts of 10-nm filaments (100 mg per 100 g of spinal cord) characterized by the absence of membranous material, a low content of tubulin and the 50 000-mol.wt.-protein component, and a high content of neurofilament peptides. Thus, the presence of tubulin in 10-nm filament preparations seems to be related to the contaminant membranous material and not to be linked to the interaction in vitro of tubulin or microtubules with neurofilaments, as has been suggested previously.