Two fragments, P1 and P2, which were obtained by digestion of haptoglobin by plasmin remained associated in neutral and alkaline buffers, although the electrophoretic and ultracentrifugal behavior of the complex was significantly different from that of native haptoglobin. The P1–P2 complex bound hemoglobin yielding a product indistinguishable at neutral pH from the normal haptoglobin–hemoglobin complex. P2 had no hemoglobin-binding property. There appears to be a weak interaction between Hb and P1 as shown by lowered electrophoretic mobility of P1 in presence of hemoglobin. However, no evidence for a stable complex between P1 and hemoglobin was obtained.