Nocardia delipidated cell mitogen (NDCM), a particulate fraction prepared from Nocardia opaca, injected i.p. in an oil/water emulsion to F6 rhabdomyosarcoma-bearing rats, inhibited the development of pulmonary metastases; 6 out of 10 rats were protected. Repeated i.p. administration of emulsified NDCM and of two other compounds, a Nocardia water soluble mitogen (NWSM a hydrosoluble fraction) and purified cell walls (CW, an insoluble macromolecular fraction) in Lewis lung carcinoma (LLC)-bearing mice resulted in a significant reduction of lung metastases. The efficiency of these fractions was enhanced by association with monokines. A combination regimen of NDCM, NWSM, and CW (100 micrograms/0.1 ml) and monokines (0.1 ml), injected i.p. in LLC-bearing mice, yielded a greater antimetastatic effect than either therapy alone. Peritoneal macrophages from mice which had been injected i.p. with NWSM or CW, when triggered either by TPA (tetradecanoyl phorbol acetate) or by zymosan, released large quantities of hydrogen peroxide and had a high rate of glucose consumption. These macrophages were activated as judged by their cytostatic activity against syngeneic P815 mastocytoma growth; they expressed biochemical markers which have been reported to characterize the activated state. Incubation of thioglycollate-elicited peritoneal macrophages with NWSM, and monokines for 72 h resulted in a cytotoxic activity against labeled LLC cells; addition of macrophage activating factor significantly increased the cytotoxic capacity of these macrophages. In view of this we postulate that the antimetastatic effect of soluble and insoluble N. opaca fractions and monokines might be mediated by activated peritoneal macrophages.