Plasma membranes from a CHO cell line, CHRC5, which exhibits multidrug resistance was studied using radiation inactivation analysis. The P-glycoprotein content of the membrane was determined by Western blots. Irradiation resulted in the loss of P-glycoprotein. The dependence of this loss on radiation dose corresponded to a target size of 250 kDa which is the molecular mass of a dimer of the P-glycoprotein. This is strong evidence to indicate that the P-glycoprotein self associates in the membrane.