In a 2-year-old girl showing purpura on her legs after administration of antibiotics, marked prolongation of activated partial thromboplastin time(APTT) and prothrombin time was noted. Circulating anticoagulants were demonstrated by the failure to correct APTT on neutralization test. A lupus anticoagulant(LA), one of the circulating anticoagulants, was detected by rabbit brain phospholipid neutralization procedure and platelet neutralization procedure. On crossed immunoelectrophoresis, the abnormal prothrombin peaks with reduced electrophoretic mobilities were considered prothrombin/prothrombin-antibody complexes because pretreatment with anti-human IgG serum caused their disappearance. This prothrombin-antibody seemed to be another circulating anticoagulant. The anti-prothrombin-antibody reduced prothrombin activities in the circulating blood of the patient to 20 approximately 30% of normal and the condition persisted for two weeks resulting in the purpura which occurred during that period. After two months, APTT was restored to normal following the disappearance of LA.