The binding of soluble human FN to MK. Guinea pig MK were incubated with human plasma FN in solution at 37°C for 2 hours. The MK were then allowed to settle on FN-coated coverslips immunostained with antihuman FN antibody, IST-4, which does not cross-react with guinea pig FN, or an anti-FN EIIIb antibody, which cross-reacts with guinea pig FN. After incubation with an FITC-conjugated second antibody, immunostained MK were visualized by confocal microscopy. The binding of soluble human FN to guinea pig MK was limited to patches on the MK surface as shown in (A). (B) shows endogenous guinea pig FN throughout the MK cytoplasm. (C) is a control in which MK were incubated with nonimmune normal IgG.