Inhibitors in some bovine ser a, active against Poliovírus type 1, strains CHAT and Mahoney, have been examinated in Al (HO)- gel sistem. The inhibitor belongs to the 19-S macro-globulins as could be shown by Sephadex-200 gel filtration, fractionation through DEAE-Celulose and centrifugation in sucrose gradient. Besides that the inhibitor could be destroyed through treatment with 2-Mercapto-Ethanol. The inhibitor combines with the virus surface in absence of cells. The complex virus-inhibitor was precipitated by the use of anti-bovine-globulin serum; the complex virus-inhibitor dissociates by the same pH value as the complex virus-19 S antibody. The author was able to show that by the neutralization of Poliovírus with bovine serum, in a cell system, the called "rest-infectiosity" (about 10%) represents a partial dissociation of virus particles which have been first bound to the inhibitor. Resistent strains against specific bovine sera have been obtained. These strains, however, were still sensible to other bovine sera, as well as to 19-S and 7-S Polio antibodies. Through absorption experiments the presence of mono-specific activity of certain bovine sera, against the strains CHAT and Mahoney of Poliovírus has been shown