Hyperfine structures in the Sb IV spectrum have been investigated. Analysis of the hyperfine patterns shows that the I-values of Sb121 and Sb123 are [Formula: see text] and [Formula: see text] respectively, in agreement with those deduced by Badami from Sb II. The magnetic moments of Sb121 and Sb123, calculated from the hyperfine separations in Sb IV, are 4.0 and 3.2 proton magnetons respectively. For antimony the ratio of the g(I) factors is g(I)121/g(I)123 = 1.82 ±.02. The application of the Landé g(I) formula to this accurate ratio leads to a value gs = 3 for the magnetic factor of the proton spin, instead of gs = 4 as found by Landé. The value gs = 3 gives slightly better general agreement between the observed and the calculated g(I) factors than is obtained for gs = 4. However, since no specific value of gs will satisfy the accurate ratio criterion for all the Zodd, Modd isotopes to which the test can be applied, it is concluded that the theory is incomplete.