The whole body retention ofcobalt after intravenous injection of carrier-free 58C0Cl, in saline solution has been measured in 16 men. The retention data for each subject have been analysed statistically and the best descriptions of the data are provided by equations with five exponential terms. Investigation levels for the radioisotopes of cobalt have been calculated for each subject. The average investigation level for G°CoCl, computed from the data is an uptake of 1.2 pCi, which may be compared with the value of 24 pCi given in ICRP Publication 10. The coefficient of variation of the radiation dose to various members of the working population after a given uptake of radiocobalt was estimated to be 20%.