Complete immunoglobulin heavy chain (IgH) genes (gamma and mu) containing the intronic IgH enhancer and mutations in the upstream promoter region were constructed in vitro and introduced into murine J558L myeloma cells by protoplast fusion. S1-nuclease mapping experiments demonstrated that IgH gene expression was extremely sensitive to mutation in an upstream region containing the octanucleotide sequence ATGCAAAT. Significant IgH mRNA levels were detected in RNA from cells transfected with IgH gene constructs in which all upstream sequences on the 5' proximal side of this element were deleted. Similar results were obtained using the precise inverse of the IgH octamer, which is found in the upstream promoter region of immunoglobulin light chain genes. Deletion of the IgH octamer, or point mutation of adenine to guanine at position 6, resulted in the loss of correctly initiated IgH mRNA. A DNA binding factor from J558L nuclear extracts was identified that appeared to recognize the octamer on the basis of differential binding to homologous restriction fragments containing the various mutations and that bound preferentially with octamer DNA fragments derived from functional relative to nonfunctional IgH constructs. Collectively, these data suggest that the octamer element contains residues that are critical to accurate immunoglobulin gene transcription and that may serve as part of a recognition locus for nuclear factors important to B-cell-specific immunoglobulin expression.