Results of the treatment of spontaneous pulmonary metastases by immunocorrection after tumour resection in mice and the analysis of antimetastatic activity of spleen macrophages (SM) by the method of adoptive transfer are presented. The injection of cytotoxic SM, NK-cells or the use of the biological response (lipid A and MDP-PE) modifiers cause no decrease in the number of lung metastases. The SM of operated mice, as distinct from the SM of immunocorrected mice causes the metastatic spreading in lungs of recipient mice. This effect is supposed to be mediated by the suppressor macrophages secreting prostaglandin E2 (PgE2). The treatment of mice with indometacin (inhibitor of the PgE2-synthesis) in the drinking water resulted in the 7-8-fold decrease in the number of lung metastases in operated mice.