Single-step mutants of Chinese hamster ovary (CHO) cells have been isolated which are resistant to killing by the anti-mitotic drugs colchicine, colcemid or griseofulvin. Two-dimensional gel analysis showed that two mutants resistant to griseofulvin, one resistant to colcemid and one resistant to colchicine carry an alteration in the beta-tubulin subunit. Most of the remaining isolates are believed to be permeability mutants on the basis of their cross resistance to drugs which do not interefere with microtubular polymerization or function (Ling and Thompson, 1974; Bech-Hansen, Till and Ling, 1976). A reduced amount of the wild-type beta-tubulin protein remained in each of the beta-tubulin mutants, but a beta-tubulin protein with a more basic isoelectric point also appeared. Messenger RNAs coding for both wild-type and variant beta-tubulins were found in at least one mutant as assayed by in vitro translation in a reticulocyte lysate. This indicates that the altered tubulin does not arise as the result of a posttranslational modification.