Diffusion-controlled adsorption of DNA from bulk solution onto a monolayer of surface-denatured cytochrome c has been used to characterize, by electron microscopy, the size and shape of single DNA molecules from bacteriophages T3 and T1 and from bovine papilloma virus. For ionic strengths, I ⩾ 0·14, the following DNA contour lengths and sample standard deviations are found: T3, (11·6 ± 0·4) μ; T1, (16·0 ± 1·0) μ and bovine papilloma virus (circular), (2·45 ± 0·07) μ. For DNA solutions with I I , presumably by a uniform, partial unwinding of the B configuration. Within the experimental error (standard deviation = ± 4%), the DNA contour lengths are found equal, whether (a) perchlorate or phenol extraction, (b) diffusion or spreading methods, or (c) ammonium acetate or sodium chloride as added salts were used. Based on experimental and operational values of standard deviation, variability of natural contour length of intact T3 or bovine papilloma virus DNA must be less than about ± 2%, if there is any. The conformation of intact T3 DNA, as measured by the distribution of end-to-end distances with the diffusion method, has been identical, at I = 0·4, within the error limits, with the calculated theoretical distribution for random coils, with small increasing deviations at 0·15 and 0·05 ionic strength.