Formation of a three-component urinary excretion function using the data of occupational exposure cases is described. Measurement results of plutonium inhalation from two subjects over 6000 and 2500 d are used in the derivation of this function. The three-component excretion function obtained is U(t) = 0.0008e2^-0.0566(t) + 0.00009e^-20.0042(t) + 0.00004e^-20.00008(t). The function is therefore proposed as a reliable basis to make an uptake assessment for operational control for mixed class of plutonium dominated by M-class chronic exposure.