The immunomodulating and antimetastatic activity of a clinically approved low molecular weight, standardized thymic peptide (TP) preparation was evaluated in BALB/c-mice. Daily subcutaneous application of TP (7 consecutive days, 1 and 10 micrograms per mouse) upregulated counts and activity of peripheral blood cells, as measured on day 14. To check the influence of TP on experimental liver metastases (liver colonization), RAW H10 lymphosarcoma cells were intravenously inoculated into BALB/c-mice. TP (1,10,100 micrograms/mouse) was subcutaneously administered daily for 7 consecutive days starting 24 hrs after tumor cell challenge. Liver colonization was investigated on day 14 after tumor cell inoculation and demonstrated a statistically significant (p < 0.05) reduction of experimental liver metastases for TP treated mice.