General Summary and Conclusions: Nine antigens were identified in the erythrocytes of cattle by means of immune isolysins. A single antibody for each of three antigens, A, B, and C, was found in as many different immune sera. Each of two other sera contained antibodies for three substances, one of them for antigens D, E, and F, and the other for antigens G, H, and I. Reagents were prepared from the antisera with which the cells of other cattle were tested for the presence of these different characters. Two of these antigens, B and D, are probably the same substance and likewise C and F are presumably identical. A study of seven of the antigens in 104 offspring suggested that each was inherited as if controlled by a single gene. Antigens C (or F) and E are closely related and presumably are controlled by allelomorphic genes. The other five characters, A, B (or D), G, H, and I appear to be independent of the former system and also of each other. The cells of an individual contained a particular antigen only if one or both parents possessed it. This type of inheritance allows for the use of this technic in excluding certain animals as the parents of a calf whose identification is questioned.