An investigation of the quantitative content of Cd and Ti in human serum samples by proton nuclear activation (PNA) has been performed and the results are presented. The activation has been induced by a 13.5 MeV proton beam of the AVF cyclotron of the University of Milan, via a (p, n) reaction on the nuclei of the target. For the quantitative determination a known amount of a reference element has been added to the samples and comparison has been made with a standard sample containing also known quantities of the elements studied. Response linearity, reproducibility and possible contaminations have been tested.