A single-step griseofulvin-resistant mutant (GrsR-4) of CHO cells which exhibit very specific cross-resistance towards certain microtubule inhibitors showed the absence of a protein of molecular weight congruent to 200,000 (designated P5) and the concomitant presence of a new protein spot, M5, of lower molecular weight (Mr congruent to 180,000) which is not present in other cell lines. Peptide mapping studies showed that proteins P5 and M5 are related to each other and that M5 may be missing a peptide fragment present in P5. In GrsR-4 X GrsS cell hybrids, both P5 and M5 were present in equal amounts, which provided evidence against post-translation mechanisms in the origin of M5 and indicated that the GrsR-4 mutant most likely contains a nonsense mutation in the structural gene for protein P5, which causes its premature termination and leads to the formation of M5. Our studies also showed that in different Chinese hamster cell lines the two alleles of the protein P5 are nonidentical and make protein products which differ from each other in isoelectric points. It is suggested that protein P5 and its isoelectric variant P6 may constitute microtubule-associated proteins.