Earlier studies showed that overexpression of B beta fibrinogen chains, by transfection of Hep G2 cells with B beta cDNA, specifically enhanced the synthesis of all three fibrinogen chains (Roy, S. N., Mukhopadhyay, G., and Redman, C. M. (1990) J. Biol. Chem. 265, 6389-6393). To determine whether overexpression of any of the three component chains of fibrinogen affects the synthesis of the other two chains, we developed stable Hep G2 cell lines transfected with individual fibrinogen chain cDNAs. As a control, cells were also transfected with expression vector, which did not contain fibrinogen cDNA. Transfection with any fibrinogen cDNA increased the synthesis of all three fibrinogen chains but not of other plasma proteins. Hep G2 cells transfected with B beta cDNA produced 3-4-fold more fibrinogen than control cells, and cells transfected with A alpha or gamma cDNA made about 2-fold more fibrinogen. Northern blot analyses showed that levels of all 3 fibrinogen mRNAs were increased and were highest in Hep G2-B beta cells. Nuclear run-on transcription assays demonstrated that increased expression of the chains was due to increased transcriptional activity. These studies show that transcription of the three fibrinogen chains is tightly linked, and increased expression of any chain specifically leads to increased synthesis of the other two chains.