A study of human blood serum is utilized to demonstrate both the x‐ray emission caused by electrons dropping into atomic inner‐shell vacancies produced by proton bombardment and the application of such x‐ray emissions to trace element analysis. Sample preparation consisted solely of pipetting 25 μl of serum onto low‐ash filter paper. Characteristic Kαx rays from copper and zinc were detected by a Si(Li) semiconductor detector to determine trace concentrations of these essential biological metals in the blood. The copper and zinc concentrations in the blood serum sample analyzed were found to be 1.16±0.13 and 0.70±0.07 ng/μl, respectively.