The immunomodulatory and antimetastatic activity of standardized aqueous mistletoe extract (sME) was evaluated in BALB/c-mice. Regular subcutaneous (s.c.) applications (three times per week for 14 consecutive days; 2, 20, 100 and 500 micrograms per injection and mouse) up-regulated thymocyte and peripheral blood leukocyte counts in tumor-bearing mice. Tumor weight and tumor volume were significantly down-regulated after application of sME doses greater than 20 micrograms per injection. To check the influence of sME treatment on growth of experimental metastases, RAW 117 H 10 lymphosarcoma cells and L-1 sarcoma cells were intravenously inoculated into BALB/c-mice to establish liver and lung colonization, respectively. sME was regularly administered starting 24 hours after tumor cell challenge. Organ colonization was investigated on day 14 after tumor cell inoculation and demonstrated statistically significant (p < 0.05) reductions of experimental liver and lung metastases for sME-treated mice.