Tunnel-injection of spin-polarized charge carriers from ferromagnetic contacts into organic semiconductors is modeled. Tunneling matrix elements and transition rates for the two spin types are calculated using a transfer Hamiltonian. The tunneling process occurs between extended states of the contact and model "molecular" orbitals. We explore the effects of the tunnel barrier height and of the ferromagnetic contact's Fermi wave vectors on the level of spin injection. The barrier height and the majority and minority spin Fermi wave vectors of the contact have strong effects on the sign and magnitude of spin injection.