The relation between the orbital ordering and magnetic structure of the crystal LaMnO3 is investigated. The dependence of the exchange parameters on the angle Φ of the orbital structure is determined. When the isotropic exchange interaction and the single-ion anisotropy, which depends on the angle Φ and the rotational distortions, are introduced into the spin Hamiltonian, a four-sublattice structure (A X, F Y, G Z) is obtained with orientation of the magnetic moments of the sublattices near the long axis of the orthorhombic cell of the crystal in the basal plane of the crystal (A X ≫ G Z, F Y). The effect of a magnetic field on the magnetic structure and the antiferromagnetic resonance spectrum are investigated taking account of the nonequivalent, anisotropic, orbitally-dependent g tensors. The spin-flop and spin-flip transition fields are calculated.