A quantitative technique for the nondestructive visualization of nanometer scale intermolecular separations in a living system is described. A calibration procedure for the acquisition and analysis of resonance energy transfer (RET) image data is outlined. The factors limiting RET imaging of biological samples are discussed. Measurements required for the calibration include: (a) the spectral sensitivity of the image intensifier (or camera); (b) the transmission spectra of the emission filters; and (c) the quantum distribution functions of the energy transfer pair measured in situ. Resonance energy transfer imaging is demonstrated for two DNA specific dyes. The Förster critical distance for energy transfer between Hoechst 33342 (HO) and acridine orange (AO) is 4.5 +/- 0.7 nm. This distance is slightly greater than the distance of a single turn of the DNA helix (3.5 nm or approximately 10 base pairs), and is well below the optical diffraction limit. Timed sequences of intracellular energy transfer reveal nuclear structure, strikingly similar to that observed with confocal and electron microscopy, and may show the spatial distribution of eu- and hetero- chromatin in the interphase nuclei.