Complement receptor 2 (CR2) has been implicated as a regulator of B cell function. In this study, we sought to identify mechanisms that control the expression of the CR2 gene in human B cells. Dibutyryl cAMP increased the DNA-binding activity of a nuclear protein that recognized specifically a CR2 promoter-defined oligonucleotide in human B cell lines. The nuclear protein was subsequently purified from B cell nuclear extracts using a biotinylated CR2 promoter-defined oligonucleotide. Partial amino acid sequence analysis of internal peptides revealed that the 42-kDa protein belongs to a family of heterogeneous nuclear ribonucleoproteins (hnRNP). Using a set of mutated double-stranded oligonucleotides, we demonstrated that the purified protein displayed sequence specificity for the CR2 promoter-defined oligonucleotide. Like some hnRNP, this protein was found to bind to single-stranded DNA. The DNA-binding activity of the purified protein increased after in vitro phosphorylation with protein kinase A. Using a CAT reporter gene driven by a single recognition site in B cell lines, dibutyryl cAMP caused a 3-fold induction of reporter gene expression. The highest induction (6.7-fold) was achieved with a combination of dibutyryl cAMP and PMA. The involvement of the nuclear protein in regulating the expression of the CR2 gene is supported by our finding that dibutyryl cAMP increased the levels of the CR2 mRNA and CR2 surface membrane protein in human B cell lines. These data strongly suggest that a cAMP-inducible hnRNP, which can recognize a novel DNA-motif, controls the expression of the CR2 gene.