The experimental system developed in our laboratory permits the study of preneoplastic cells and events in foreign body tumorigenesis of mice. The preneoplastic process is accessible to experimentation already during the first few weeks of foreign body reaction, i.e. up to 30 months prior to actual tumor appearance. The insights that may be gained from such investigations are obviously not limited to foreign body tumorigenesis but extend to the general phenomenon of neoplastic cell transformation. Specific areas of investigation include the following: (1) Exact identification and study of preneoplastic originator cells which carry the specific information of later tumor characteristics. (2) analysis of factors which determine or influence direction and pace of preneoplastic cell "maturation" during year-long latency. (3) study of ultrastructural, subchromosomal, and biochemical-regulatory features in tumor cells with attempts to trace these features back to the preneoplastic cell in different stages of "maturation". (4) search for specific virus or viral genomes and (5) assessment of the role of immunity in the development of foreign body- induced tumors.