Normal and established human epithelial cell lines obtained from the same organs were compared for their capacity to retract a fibrin clot. Fibrin clot retraction was maximal in normal epithelial cells, reduced in established nontumorigenic lines, and lost in tumorigenic cancer cell lines. Fibrin clot retraction efficiency seemed to be related to the degree of cellular spreading within the clot at the end of the test. Previous works and the present study suggest that fibrin clot retraction is correlated with some steps of cell transformation in vitro.