The functional molecular weight of band 4.5 polypeptide was measured by applying the classical target theory to radiation inactivation data of the cytochalasin B binding. Band 4.5 polypeptides purified from human erythrocyte membranes were irradiated at -45 to -50℃ with an increasing dose of 1.5 MeV electron beam, and after thawing, cytochalasin B binding activities were assayed. Each activity measured was reduced as a simple exponential function of radiation dose. D37, dose appeared to be 6.7 mega rads, from which the target size (radiation sensitive mass) of band 4.5 polypeptide was calculated to be 95,500 daltons. This result with other informations available in literature suggests that band 4.5 polypeptide may exist as a dimer in human erythrocytes.
