The highly conserved octamer sequence ATGCAAAT or its inverse complement found in all human and murine immunoglobulin gene promoters has been demonstrated to be necessary in the lymphoid-specific transcription by deletion analysis. Trans-acting factors that interact with the octamer motif are thought to be involved in this tissue-specific expression. Using a gel mobility shift assay, we have identified both lymphoid-specific and ubiquitous nuclear factors that interact with a human gamma 1 heavy chain gene promoter region containing the octamer motif, consistent with the results obtained with murine heavy or light chain promoter regions. We have purified an octamer binding protein from human B cells by sequence-specific DNA affinity chromatography. Renaturation of gel-purified protein allowed the identification of a polypeptide with a molecular weight of 74 kilodaltons (kD) that is capable of recognizing and binding to the octamer motif. This 74 kD protein seems to be also present in T-cells and non-lymphoid cells. The possible function of the factor is discussed.