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The paper studies the magnetic properties and ferromagnetic resonance of the yttrium-lanthanum iron garnet system Y3(1 - x)La3xFe5O12 at compositions where x = 0, 0.5, 0.10, 0.20, 0.30, 0.50, 0.75 and 1.00. The single-phase region with the garnet structure determined by X-ray powder pattern analysis and metallographic observations extends to approximately 0.10 < x < 0.20. However, magnetic and ferromagnetic resonance observations indicate a somewhat narrower single-phase region. The saturation magnetic moment per gram sigma, initial permeability mu, coercive force Hc, effective g-factor g(eff) and resonance line-width delta H (at 3970 and 9160 Mc/sec) are measured as a function of the compositions. Variations in sigma with composition and the magnitude of mu as viewed from various magnetization mechanisms are discussed. From the frequency dependence of g(eff), the intrinsic g-factors and internal fields H1 of the materials are computed. Exponential dependence of delta H, Hi and Hc on x in certain composition regions observed experimentally is interpreted from the viewpoint of magnetic inhomogeneity. (Author)