Adenoviruses represent a broad group of human pathogens that currently have no specific and safe drugs for treatment. We demonstrated direct (non IFN-mediated) antiviral activity of cycloferon (10-carboxymethyl-9-acridanone, CMA), a potent interferon inducer, against adenovirus type 6 (Ad6) in Hep-2 cells. Virus production and details of morphogenesis were studied by ELISA with antibodies to the Ad6 hexon protein, and transmission electron microscopy, respectively. Immunoenzyme assay revealed that CMA does not inhibit viral protein synthesis but instead strongly reduces the ability of the virus to generate infectious progeny virus in a dose dependent manner. Ultrastructural study shows that CMA alters the structure of intranuclear virus-specific inclusions. We suggest that CMA suppresses the late stages of viral cycle in the infected cell.