Bone marrow stromal cells are requisite for the proliferation of hematopoietic cells and communicate with each other via gap junctions. Marrow stromal cells expressed connexin 43, but not connexin 32. H-1/A, a murine marrow stromal cell line, underwent adipocyte differentiation at confluence, and expressed the 3.0 kilobase mRNA species of connexin 43 before differentiation. H-1/A cells studied with the dye-transfer method showed gap-junctional communication with adjacent cells but lost this communication during differentiation. The connexin 43 transcripts in H-1/A cells were down-regulated before the expression of glycero-monophosphate dehydrogenase was induced. Loss of gap-junctional communication was regulated at the mRNA level of connexin 43. Connexin 43 expression was down-regulated at the transcriptional level.