The mammalian SWI/SNF-related complexes facilitate gene transcription by remodeling chromatin using the energy of ATP hydrolysis. The recruitment of these complexes to promoters remains poorly understood and may involve histone modifications or direct interactions with site-specific transcription factors or other cofactors. Here we report the isolation of two related but distinct cDNA clones, hOsa1 and hOsa2, that encode the largest subunits of human SWI/SNF. hOsa1 is identical to previously reported BAF250, and hOsa2 shares a high degree of sequence similarity with hOsa1. Mass spectrometric analysis, and immunoblotting with antibodies specific to hOsa1 or hOsa2 demonstrate the presence of both proteins in SWI/SNF-A but not in the related polybromo-BRG1-associated factors complex purified from HeLa cells. Co-precipitation studies indicate that hOsa1 and hOsa2 associate with BRG1 and hBRM through the C-terminal domain of hOsa. We define multiple domains within hBRM and BRG1 that interact with the hOsa C terminus. In cultured mammalian cells, hOsa1 and hOsa2 stimulate transcription by the glucocorticoid, estrogen, and androgen receptors. The glucocorticoid receptor-mediated activation is not observed with the C-terminal domain or with the hOsa2 polypeptide lacking the ARID DNA binding domain. These results suggest that hOsa1 and hOsa2 participate in promoting transcriptional activation by the steroid hormone receptors.