By following the kinetics of abortive and productive synthesis in single-round transcription assays, we confirm the existence of two general classes of initial transcribing complexes (ITCs), which we term "productive ITC" and "unproductive ITC". The productive ITCs are able to escape from the promoter rapidly to produce full-length transcripts, but only after carrying out an obligate series of abortive initiation steps. The unproductive ITCs were found to synthesize mostly abortive transcripts of 2-3 nucleotides and escape from the promoter extremely slowly, if at all. Formation of the unproductive ITC is not due to the inactive RNA polymerase. Instead, RNA polymerase molecules recovered from both the productive and unproductive ITC fractions were shown to carry out abortive and productive synthesis with both the partitioning tendency and transcription kinetics similar to those of the original enzyme. Our results suggest that early transcription complexes are partitioned into the productive and unproductive ITCs most likely during the formation of open promoter complexes. The extent of partitioning varies with individual promoter sequences and is dependent on the nature and concentration of the initiating nucleotide. Thus, multiple classes of ITCs can be formed during promoter binding and transcript initiation.