Tubulin was isolated by a combination of affinity (ethyl N-phenylcarbamate-Sepharose) and ion exchange (DEAE-Sephacel) chromatography from mung bean and cultured carrot suspension cells. SDS-PAGE (Blose 1981) of mung bean tubulin has shown it to consist of two major subunits (MBT 1 and MBT 2) and a minor subunit (MBT 3). Tubulin isolated from carrot cells was resolved into only two bands on SDS-PAGE (slow moving subunit was named CT 1). However, the faster moving subunit on SDS-PAGE was resolved into two bands (CT 2 and CT 3) on SDS-4M urea-PAGE. On SDS-4M urea-PAGE, CT 1 migrated faster than CT 2, CT 3. By contrast in SDS-4M urea-PAGE, mung bean tubulin remains unresolved. Mammalian tubulin could be resolved into α and β-subunits in both electrophoretic systems. Monoclonal antibodies to mammalian α and β-tubulin subunits (MCA-Tα and MCA-Tβ, respectively) and Western blot analysis clearly demonstrated a cross-reactivity of MCA-Tα with MBT 2, MBT 3, CT 2 and CT 3, while MCA-Tβ showed cross-reactivity with MBT 1 and CT 1. Although MBT 2, MBT 3, CT 2 and CT 3 are immunologically related to the α-subunit of mammalian tubulin, their migration on SDS-PAGE was reversed with respect to MBT 1 or CT 1, which were immunologically related to the β-subunit of mammalian tubulin. Peptide mapping patterns also supported above the results.