An x-ray fluorescence system has been designed to make elemental analyses of trace elements in low-Z matrices. It consists of radioisotope exciters, a Si(Li) detector, and a computer-based pulse height analyzer. Qualitative measurements of all elements heavier than phosphorus (Z = 15) and quantitative measurements of all elements heavier than potassium (Z = 19) can be made. For samples fluoresced for 1000 s on 9 mg/cm2filter paper, the three sigma detection limits range from 10 to 100 ng/cm2. Accuracy is within ±10% and reproducibility is better than 5%. The method often requires no sample preparation and is completely nondestructive. Quantitative information obtained with such a system is useful in monitoring trace element concentrations in air, water, or biological material.