We have recently identified and characterized pancreatic stellate cells (PSC) in rats and humans (Gastroenterology 1998, 15:421–435). PSC are suggested to represent the main cellular source of extracellular matrix in chronic pancreatitis. Now we describe a paracrine stimulatory loop between human macrophages and PSC (rat and human) that results in an increased extracellular matrix synthesis. Native and transiently acidified supernatants of cultured macrophages were added to cultured PSC in the presence of 0.1% fetal calf serum. Native supernatants of lipopolysaccharide-activated macrophages stimulated the synthesis of collagen type I 1.38 ± 0.09-fold of control and c-fibronectin 1.89 ± 0.18-fold of control. Transiently acidified supernatants stimulated collagen type I and c-fibronectin 2.10 ± 0.2-fold and 2.80 ± 0.05-fold of control, respectively. Northern blot demonstrated an increased expression of the collagen-I-(α-1)-mRNA and fibronectin-mRNA in PSC 10 hours after addition of the acidified macrophage supernatants. Cell proliferation measured by bromodeoxyuridine incorporation was not influenced by the macrophage supernatants. Unstimulated macrophages released 1.97 pg TGFβ1/μg of DNA over 24 hours and lipopolysaccharide-activated macrophages released 6.61pg TGFβ1/μg of DNA over 24 hours. These data together with the results that, in particular, transiently acidified macrophage supernatants increased matrix synthesis, identify TGFβ as the responsible mediator. In conclusion, our data demonstrate a paracrine stimulation of matrix synthesis of pancreatic stellate cells via TGFβ1 released by activated macrophages. We suggest that macrophages might play a pivotal role in the development of pancreas fibrosis.