Information on the biokinetics of cerium can be obtained directly from humans by using stable isotopes as tracers. Neutron, photon and proton activation analysis have all been tested as analytical techniques able to quantify different isotopes of the same element in biological fluids. The experimental conditions were optimized for Ce analysis in blood plasma samples. The performances of the different techniques have been explored. The simultaneous determination of two Ce isotopes with the required sensitivity in the order of few ppb is difficult to obtain using a single technique, and, therefore, a combination of techniques can be envisaged.