In this report, we describe the isolation of a 50 kDa polypeptide from the detergent-resistant cytomatrix of unfertilized sea urchin egg. This polypeptide shares with the intermediate filaments the property of insolubility in high ionic strength buffer solution. However, it does not cross-react with anti-vimentin and anti-cytokeratin antibodies. Studies performed by indirect immunofluorescence microscopy with an immunospecific serum raised against this polypeptide show that during the first cell cycle the polypeptide exhibits similar configuration changes as those described for tubulin. Using immunocytochemical light and electron microscopy, we present evidence indicating that this 50 kDa polypeptide is a constituent of the isolated mitotic apparatus; it is mainly located on patches of microfibrillar material found close to the microtubules. The 50 kDa polypeptide is not extracted from taxol-assembled microtubules by the 0.6 M NaCl treatment. However, the difference in solubility between this protein and the previously studied microtubule-associated proteins does not preclude the possibility of the 50 kDa polypeptide on being a "microtubule-associated protein". The possible significance of this novel cytoskeletal component is discussed.