Microfocal projection radiography was used to examine a new model of experimentally induced inflammatory arthritis, developed at Roche Projects Ltd. The high resolution and magnification of the stereoprojection radiographs provided a non-destructive and three-dimensional assessment of the changes in the arthritic knee joints and enabled direct measurements to be made from the radiographs. During the time course study it was possible to quantify the increase in the size of the suprapatella bursa in the early stages of the inflammatory process, the onset and development of erosions in the articular bones and both juxta-articular and systemic osteoporosis. The results of the quantitative radiographic study when compared with standard arthritis assay techniques of knee diameter and flexion showed maximum damage when the latter were near normal. Histology confirmed the radiographic results, although, alone it did not contribute substantially to the assessment of the changes occurring in the diseased joints. The measurements from the projection radiographs showed that the destruction of the articular bone progressed rapidly subsequent to subperiosteal erosions and consequent invasion of pannus into the epiphyses. The rate and degree to which articular bone was destroyed was independent of the extent to which juxta-articular and systemic osteoporosis had developed. Microfocal projection radiography has considerable advantages over any other method in quantifying the changes in arthritic joints for evaluating the pathogenesis of the arthritis and in screening different compounds.