Abstract: A procedure is disclosed for determining optimal doses, scheduling, routes of administration, the time for initiation, the magnitude and the duration of the anti-tumor effects, at various times following the administration to laboratory test animals such as rodents of doses of reticulo-endothelial stimulants by convenient anatomical routes. Following administration of the reticuloendothelial stimulant material the animals are challenged intravenously with radiolabelled tumor cells, and after 1 to 24 hours they are sacrificed, the lungs harvested and counted for radioactivity. The method allows for comparative studies on a number of agents and can be used as a screening assay as well as for the quality control of existing products.