A semi-automated image analyzing system was developed for the quantitative study of the in vitro osteoclast activity. It is able to determine the percentage of the matrix reabsorbed by cultured osteoclasts. It counts and sizes the worn areas and displays the results together with two-color coding useful for visual interpretation. A specific semi-automated procedure was designed to simulate human sight. It detects local gray level differences while matching with the surrounding gray levels. It implements routine calculation enabling the operator to continuously control the process. We were able to accurately and precisely quantify the reabsorbed percentage of meaningful fields in a slice. This image-analyzing system is useful in the study of physiological or pathological osteoclast activity, in various diseases involving bone, such as osteoporosis, Paget's disease of bone or neoplastic conditions, and in the evaluation of osteoclast response to various agents.