Reproducible images of DNA and the interaction of RNA polymerase with supercoiled DNA on mica have been obtained by scanning force microscopy (SFM) in air. This technique allowed visualization of the assembly of distinct nucleoprotein complexes at specific sites which initiate DNA replication. Transmission electron microscopic controls were performed on the carbon-coated replica of the same specimens previously scanned by scanning force microscope. Image contrast formation has been investigated and lateral friction forces have been qualitatively characterized with SFM on DNA-covered samples.