Class III genes require multiple cellular factors for transcription by RNA polymerase III; these genes form stable transcription complexes, which in the case of Xenopus 5S genes are correlated with differential expression in vivo. The minimal number and identity of the factors required to form both stable and metastable complexes on three class III genes (encoding, respectively, 5S RNA, transfer RNA, and adenovirus VA RNA species) were determined. Stable complex formation requires one common factor, whose recognition site was analyzed, and either no additional factors (the VA gene), a second common factor (the transfer RNA gene), or a third gene-specific factor (the 5S gene). The mechanism of stable complex formation and its relevance to transcriptional regulation were examined in light of the various factors and the promoter sequences recognized by these factors.