Fibrinoid material from human placental villi was investigated employing histochemical methods. According to the present results, the fibrinoid material seems to consist of glucoproteins containing a neutral mucopolysaccharide and a protein.This protein contains arginine, tyrosine, tryptophan and cysteine. The fibrinoid material displayed histochemical reactions very similar to those of fibrin. However, the insensity of the reactions was always higher in the fibrinoid substance. Histological staininng appears to be more effective in differentiating fibrinoid from fibrin