We present a technique allowing interactive visualization of large, scalar, discrete volume fields as semi-transparent clouds "on the fly", i.e., without preprocessing. Interactivity is not restricted to geometric transformations, but includes all possible methods of processing the data (thresholds, filtering, clipping, illumination, etc.). Our system flexibility trades-off quality for performance at any desirable level. In particular, by using a scanline based method and a DDA-based traversing scheme instead of ray-tracing we achieve real-time processing during previewing. By means of the "pyramidal volume" traversing technique we achieve high-quality, constant-time filtering independent of the data resolution. Several filters help to detect "fuzzy", obscured hot spots, even within noisy data. Our visualization stages, maximizing thereby flexibiblity. Four different immunination models have been implemented. Our method can be used in numerous areas, including 3-D medical imaging, geol ogic, technical, and environmental applications.