Antitumor agents of a new structural type are reported. A number of cyanines with mono-, di- and tricyclic nuclei, merocyanines and oxonols have been i. p. screened for antitumor activity against P388 leukemia and B16 melanoma. Among these compounds, monomethin-, trimethin- and pentamethincyanines having naphthothiazole, naphthoxazole and benzindole nuclei resulted in a significant prolongation of the survival time of tumor-bearing mice. Replacement of the conjugated chain system between the terminal two nuclei with a saturated aliphatic chain produced a marked decrease in the antitumor activity. Structure-activity relationships are discussed.