Detection of low-affinity binding sites for uPA on Nalm6 cells. Cells were incubated with uPA and bound, active enzyme measured in plasminogen activation assays (•). Parallel incubations without cells were included to determine uPA carryover into activity assays (+). Specifically bound uPA (▴) was calculated by subtraction of nonbound activity and data fitted to a single binding isotherm to determine the K_d for uPA binding to Nalm6. Values are expressed as mean ± SE, n = 3.