Src is required for u-PA–dependent cell shape changes. Mouse 3T3 cells from Src^−/− and wild-type mice were transfected with a human u-PAR cDNA. u-PAR–positive cells (determined by direct binding and immunofluorescence assays) were then supplemented with 10 nmol/L ATF (the amino terminal fragment of human u-PA) and incubated for 0 or 30 minutes at 37°C; at the end of the incubation, the cells were washed, stained with phalloidin (A, C, E, G, I, and K) or with antivinculin antibodies (B, D, F, H, J, and L), and viewed and photographed under the immunofluorescence microscope. From (A) to (F): Src+ cells incubated in the absence (A and B) or in the presence of 10 nmol/L ATF for 30 minutes (C, D, E, and F). From (G) to (L): Src^−/− cells incubated in the absence (G and H) or in the presence of 10 nmol/L ATF for 30 minutes (I, J, K, and L).