We have developed a new progressive technique that allows real-time volume rendering of large tetrahedral meshes. Our approach incrementally streams portions of the mesh to a hardware-assisted volume renderer and displays approximations that increase in quality until a full-quality rendering is achieved. The results of previous steps are stored as image data and re-used in each subsequent refinement, thus leading to an efficient rendering. Our interactive representation of the dataset is efficient, light-weight, and high quality. The result is a level-of-detail framework for interactive exploration of tetahedral datasets too large to render using conventional approaches.