Radioligand binding studies using ¹²⁵I-uPA with U937 cells and Nalm6 cells. A low concentration range (up to 4 nmol/L) ¹²⁵I-uPA was incubated with U937 or Nalm6 cells, at 10⁷ cells/mL. With U937 cells, bound and free radioactivity were separated by centrifugation through sucrose solution and bound uPA counted (•). Nonspecifically bound radioactivity was determined in the presence of excess cold uPA (▪) and specifically bound counts were calculated (▴). Values are shown as mean ± SE, n = 4. Specific binding to Nalm6 cells was negligible as shown by the overlap of the response with ¹²⁵I-uPA (◊) and¹²⁵I-uPA plus cold uPA (+).