Antibacterial, antifungal and antitumor activity of 15 newly synthetized compounds were investigated in vitro. These compounds were mainly 1-[4-pyridyl]-pyridinium salts, dipyridyl sulphides and corresponding sulphones. Antitumor activity was evaluated in a model of leukemia L1210, P388, Sarcoma 180, Ehrlich carcinoma, NK-lymphoma and melanoma B16. None of the compounds investigated, even at the concentration of 100 micrograms/ml, appeared to influence Gram-negative bacteria. However, at the concentration as low as 1.0 microgram/ml, three of them (No. III, VI and VII) strongly inhibited the growth of Gram positive bacteria and fungi. The compounds did not prolong the survival time of Sarcoma 880 leukemia L1210 or P388--bearing mice. Few of the compounds examined exerted marked antitumor effect on Ehrlich carcinoma and NK-lymphoma--compounds No. I and XV inhibited the growth of these tumors by ca. 70%. The strongest inhibitory effect on melanoma B16 was observed with compound No. XV, the effect being dose-dependent.