Highly purified human fibrinogen was labelled with radioactive iodine and its interaction with cultured human embryo lung fibroblasts (MRC5) was examined. The cell monolayer was incubated at 37 degrees C with 125I-fibrinogen in phosphate/saline buffer containing 1 mM Ca2+ and 1 mM Mg2+. A direct interaction between 125I-fibrinogen and MRC5 was observed. The binding was time-dependent, reached saturation at 10 min and was regulated by the density of the cell monolayer. Non-labelled fibrinogen inhibited the interaction but unrelated proteins, including fibronectin, ovalbumin or myoglobin, did not. Monospecific Fab fragments, directed to fibrinogen, inhibited binding by 55.3% at a 50/1 molar ratio while non-immune Fab produced a 1.5% inhibition at similar concentration. Autoradiography of the display of fibroblast-bound 125I on a 7.5% polyacrylamide gel showed that the extract exhibited electrophoretic bands characteristic of the constitutive B beta and gamma chains of the fibrinogen molecule. An apparent affinity constant, Ka = 6.7 +/- 0.2 X 10(6) M-1, was estimated from binding isotherms. After a 30-min incubation time the interaction between 125I-fibrinogen and the cells was completely reversible and displaceable by a large molar excess of non-labelled fibrinogen. When compared to fibroblasts (MRC5 or W138), cultures of human embryo epithelial cells (EUE) failed to interact with 125I-fibrinogen, providing evidence for the specificity of binding for fibroblast monolayers. Plasmin-degradation fragments D and E, of 100000 and 50000 relative molecular mass respectively, were tested for their capacity to inhibit fibrinogen binding. At a 1/400 125I-fibrinogen/fragment molar ratio, fragment E inhibited binding by 30% while fragment D produced a 3% inhibition only. Altogether, the results demonstrate that human fibroblasts possess specific binding sites for fibrinogen, which exhibit the characteristics of a receptor system regulated by the culture state of the cells. In addition the structural features, which are necessary for the interaction of fibrinogen with the cells, are probably located in the E domain of the molecule.