Cryoelectron micrographs of purified S-PM2 phage particles. (A) Showing one phage particle in the extended form and one in the contracted form both still have DNA in their heads and (B) Two phage particles with contracted tail sheaths, the particle on the left has ejected its DNA. The lack of collar structure is particularly visible in (B). The diameter of the head is 65 nm. Pictures were taken at the University of Warwick with the kind assistance of Dr Svetla Stoilova-McPhie.