The effect of Thuja occidentalis extract on the inhibition of lung metastasis induced by B16F-10 melanoma cells was studied in C57BL/6 mice. The extract was administered by three different modalities. A remarkable reduction in tumor-nodule formation was shown by simultaneous (74.4%) and prophylactic (71.5%) mode of administration. The effect was comparatively low in drug administration after tumor development (60.2%). Increased lung collagen hydroxyproline (21.13 microg/mg protein) in the metastasized lungs of control animals compared with normal animals (0.98 microg/mg protein) was significantly reduced in Thuja-treated animals. The elevated level of uronic acid (349.5 microg/100 mg tissue) and hexosamine contents in metastatic control animals was significantly reduced in the animals treated with alcoholic extract of Thuja. Similarly the elevated levels of serum sialic acid and serum gamma glutamyl transpeptidase activity in the untreated control animals was significantly reduced in the animals treated with the extract of Thuja. The lifespan of the Thuja treated animals also was seen to be significantly increased.