Cyanocycline A was cytotoxic against Meth A cells in vitro, and also showed marked activity against the same cell line grown as an experimental ascites tumor. This antibiotic inhibited nucleic acid synthesis in Escherichia coli and Meth A cells. The antimicrobial activity of the antibiotic was reversed by addition of exogenous herring sperm DNA. Cytofluorometric analysis of cyanocycline A-treated Meth A cells showed an unusual pattern of the relative content of DNA and RNA per cell. These results suggested that cyanocycline A binds to DNA, and that it inhibits nucleic acid synthesis.