This study examines the synthesis and cytostatic activity of new 5,6-dimethyl-1-substituted-6H-pyrido[4,3-b]carbazole derivatives. Their structures were confirmed by (1)H-NMR and elemental analysis. Seven of the new compounds were tested by the SRB method in vitro against human lung cancer (A549) and human kidney cancer (A498) cell lines. Biological tests indicated remarkable cytostatic effects of four compounds tested in comparison with ellipticine and cisplatin as reference drugs. One particular compound 3c was about four times more active on A498 than ellipticine with similar activity on the A549 cell line, and outperformed cisplatin activity on both tumor cell lines.