To observe extended DNA in specimens negatively stained with uranyl acetate, cytochrome c has been bound to the DNA and the resulting complex has been negatively stained. DNA strands are revealed by an accumulation of negative stain around the DNA-bound cytochrome c. The DNA—cytochrome c complex is sometimes observed as an electron-transparent thread, usually 4.0 to 6.5 nm in diameter, within the negative stain. The length per nucleotide of duplex DNA from bacteriophage πX174 is 0.29 nm. The above technique has been used to observe complexes of bacteriophage T7 DNA with T7 capsids produced by (a) osmotic shock of bacteriophage T7 and (b) reaction of bacteriophage T7 with glutaraldehyde. Some of the latter DNA—capsid complexes had DNA partially ejected, and a core within the capsid was sometimes visible.