A novel Ets protein was isolated by yeast one-hybrid screening of a cDNA library made from lipopolysaccharide-stimulated mouse splenic B cells, using the SP6 kappa promoter kappaY element as a bait. The novel Ets protein was most closely related to PU.1 and Spi-B within the DNA binding Ets domain and was therefore named Spi-C. However, Spi-C may represent a novel subgroup within the Ets protein family, as it differed significantly from Spi-B and PU.1 within helix 1 of the Ets domain. Spi-C was encoded by a single-copy gene that was mapped to chromosome 10, region C. Spi-C interacted with DNA similarly to PU.1 as judged by methylation interference, band-shift and site selection analysis, and activated transcription of a kappaY element reporter gene upon co-transfection of HeLa cells. Spi-C RNA was expressed in mature B lymphocytes and at lower levels in macrophages. Furthermore, pre-B cell and plasma cell lines were Spi-C-negative, suggesting that Spi-C might be a regulatory molecule during a specific phase of B lymphoid development.