To examine the functional organization of the human triosephosphate isomerase (TPI) promoter, deletion, insertion, and linker scanning mutations were introduced into the TPI promoter of hybrid TPI/beta-globin genes. These genes were transiently expressed in mouse L and human HeLa cells, and the effect of each mutation on the frequency and position of transcription initiation was assayed by S1 nuclease transcript mapping. Multiple positive regulatory elements reside between positions -595 and +1 in L cells and -920 and -7 in HeLa cells and coordinately promote maximum hybrid gene transcription. These elements include an array of GC boxes (positions -126 to -48) that variably conform to the consensus Sp1-binding site, and a canonical TATA box (positions -27 to -21) that is essential for detectable levels of transcription. In an additive yet position-dependent fashion, the GC boxes function in cis to the TATA box to control both the frequency and position of transcription initiation. Additional positive elements reside upstream of position -131 and are required for full promoter function. Also, an inhibitory element(s) residing between position -7200 and either -2800 in L cells or -920 in HeLa cells reduces transcription approximately 7-fold relative to the level of transcription achieved with the maximally active promoter.