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We have devised a method to follow the stability of individual ternary transcription complexes containing Escherichia coli RNA polymerase halted at many different sites along a DNA template during the transcription process. Studies of complexes formed with phage T7 DNA templates reveal at least three general classes of ternary complexes that differ dramatically in their properties. Complexes of one sort (normal complexes) are highly stable to dissociation and denaturation under a variety of solution conditions. They remain intact and active for up to 24 hours even in salt concentrations up to 1 m-K⁺. This suggests that they are stabilized to a significant extent by non-ionic interactions between RNA polymerase and the nucleic acids. We consider these to be the normal complexes formed during RNA chain elongation. Complexes of a second sort (release complexes) dissociate rapidly, releasing free RNA transcripts …