Antitumour, antileukosis and antimetastatic effects of lipopolysaccharide (LPS) isolated from Pseudomonas solanacearum have been studied. It is established that LPS does not possess the antitumour effect on experimental animals with Lewis lung carcinoma, melanoma B-16 and sarcoma S-37 and vice versa, intensifies the tumour growth. The life time of animals with experimental leukoses lymphocytic leukemia P-388 and lymphoid leukemia L 1210 inconsiderably increases. At the same time LPS possesses the expressed antimetastatic effect that has manifested in the decrease of the volume (40 and 5 times) and of the amount (4-4.2 times) of metastases in mice with Lewis lung carcinoma and melanoma B-16, respectively. Study of the contribution of certain structural components of LPS molecule to the total biological activity has shown that O-specific polysaccharide and oligosaccharide of core take the expressed antimetastatic effect. Lipid A in the used dose weakly modified the development of Lewis lung carcinoma metastases.