We developed two new monoclonal antibodies, designated NNKY3-2 and NNKY4-7, that recognized a 40-kD platelet protein. They appeared to be monoclonal anti-Fc gamma receptor II (Fc gamma RII) antibodies from the results of flow cytometric binding inhibition studies using another monoclonal anti-Fc gamma RII antibody (2E1). The addition of NNKY3-2 or NNKY4-7 to platelet-rich plasma (PRP) led to a typical aggregation pattern preceded by a lag phase, but their addition to washed platelets did not induce aggregation. The aggregation of PRP by these antibodies was inhibited by prostaglandin E1 (PGE1) or staurosporine (protein kinase C inhibitor), whereas it was only slightly affected by a monoclonal anti-GPIIb/IIIa antibody or Arg-Gly-Asp-Ser. Furthermore, these antibodies induced the aggregation of washed platelets plus normal serum, but not that of washed platelets plus heat-treated serum (destruction of complement activity). These results suggest that NNKY3-2 or NNKY4-7-induced aggregation involves an unusual pathway independent of fibrinogen, and that the important factor is the participation of complement. NNKY3-2 and NNKY4-7 may be useful to study the relationship between autoantibodies, the Fc receptor, and complement in idiopathic thrombocytopenic purpura.