A fatal hemorrhagic syndrome accompanied by marked hypoprothrombinemia was produced in young rats by admn. of 2-(3-cyclohexyl-propyl)-3-hydroxy-l,4-naphthoquinone, 2-(2-methyloctyl)-3-hydroxy-l,4-naphthoquinone and 2-[3-(decahydro - 2- naphthyl)propyl] - 3 - hydroxy - 1,4 - naph-thoquinone. These compounds produced only minimal pathologic changes in the liver, did not destroy prothrombin in vitro, and possessed only slight bacteriostatic activity against Escherichia coli, the principal source of vit. K in the gut. The toxic properties of the compound listed first above were antagonized partially by 2-methyl-l,4-naphtho-quinone and completely by vit. K1. It is postulated that these 3-hydroxynaphthoquinohes compete with the K vits. in processes essential to prothrombin formation and by displacing these vits. produce the hemorrhagic syndrome.