A flow-stretching assay was employed to elucidate polycation/DNA interactions at the single-molecule level. Bacteriophage lambda-DNA (48.5 kbp) was attached at one end to a PEG-modified glass surface and stretched by buffer flow. The stretched DNA had an approximate length of 12 mu m. Upon injection of polylysine homopolymer, the DNA folded into a globule structure within a few seconds. Injection of a polylysine graft copolymer having hydrophilic dextran side chains also induced collapse of the stretched DNA. In comparison to these polymer, a copolymer with higher graft content did not cause DNA collapse but rather caused 25% shrinkage of the extended DNA. These results were compared with those observed with unstretched DNA.