Subclass characterization of anti-tPA immunoglobulins. The immunoglobulins bound to immobilized tPA were revealed using mouse subclass–specific monoclonal antibodies against human immunoglobulins in an ELISA system. The results are expressed as absorbance at 492 nm (mean values ± SE of 3 experiments) and indicate that the anti-tPA antibodies of patient no. 1 are IgG1, whereas those of patient no. 2 are IgG3. No significant signal was obtained from the immunoglobulins of a healthy control. OD indicates absorbance.