SWAP‐70 is part of a protein complex that catalyzes cell‐free DNA recombination between immunoglobulin heavy chain gene switch region substrates. This report studies the expression pattern of SWAP‐70 in mouse tissues, sorted cells, and cultured primary cells. SWAP‐70 RNA is strongly increased upon switch‐induction of spleen cells, and very weakly expressed in thymus and bone marrow. SWAP‐70 protein is specifically expressed in B cells, and levels increase rapidly after stimulation. Tissue staining shows strong expression in germinal center B cells, while macrophages and T lymphocytes do not stain. SWAP‐70 is not detected in early B cells in the bone marrow. Its expression during mouse ontogeny after birth correlates with the appearance of non‐IgM isotypes. While SWAP‐70 localizes to the cell nucleus in activated B cells, it is not tightly associated with the chromatin and is found in the cytoplasm as well. SWAP‐70 expression is not increased by γ or UV irradiation of spleen cells, nor does it depend on p53. These characteristics are consistent with the putative role of SWAP‐70 in immunoglobulin class switching.