CD40 is a surface glycoprotein, member of the nerve growth factor receptor family, which is expressed on B cells and plays an important role in their development, growth, and differentiation. Using chromosomal in situ hybridization, we localized the CD40 gene to the long arm of chromosome 20, bands q12-q13.2. This localization correlates well with the mapping of the murine CD40 gene to the distal region of chromosome 2, syntenic to the human 20q11-q13 region.