Transfection of foreign DNA into eukaryotic cells has become an important tool in molecular biology. Based on the results of previous studies of the core structure of human adenoviruses, we have developed a novel transfection method. The procedure involves the in vitro reconstitution of foreign DNA-of viral or other origins-with the major core protein VII of adenovirus type 2 (Ad2) or protamine from salmon sperm. Both proteins are rich in basic amino acids and appear to share structural features. The DNA-protein complexes are added directly to the medium of eukaryotic cells. The in vitro formation of specific DNA-protein complexes can be assessed by gel electrophoretic analyses. Bovine serum albumin does not enter into specific complexes with DNA. Transfection of DNA-protein VII or DNA-protamine complexes results in their rapid transport into the cell nuclei. About 2-4 hr after transfection, up to 40% of the DNA added to cell cultures in complexes can be found in the nucleus, as compared with less than 10% of the DNA when other transfection methods are applied or when naked DNA is added to cell cultures. DNAs transfected by the new method into mammalian or insect cells retain their characteristic restriction patterns at least 48 hr after transfection and are expressed efficiently. Supercoiled circular plasmid DNAs are converted to open circular or linear DNA. Expression has been measured both for transiently expressed genes (chloramphenicol acetyltransferase gene, Ad2 DNA in human HeLa cells) and for genes that have been integrated into the host genome and are expressed permanently, such as the gene for neomycin phosphotransferase in hamster BHK21 cells.(ABSTRACT TRUNCATED AT 250 WORDS)