After the suggestion that heavy deformed even-even nuclei should be superfluid (~), the energy gap of infinite nuclear matter has been estimated by many authors (8.4) for different nucleon-nucleon pair states, and has been found to be of the order of 0.1MeV or smaller at (~normal~ density corresponding to a Fermi momentum k F = 1.4 fm -~. We consider this result unsatisfactory for the reason we are going to explain. Our analysis will lead us to consider a neutron-proton pairing, which should be a characteristic feature of deformed nuclei. The nucleon-nucleon interaction is able to bind the proton-neutron system, and this makes the perturbative expansion for the energy of any N-body system divergent (5). For an infinite system, in particular, the Brueckner-Goldstone perturbative expansion is divergent (6.7) (a fact that may escape notice either because numerical computations are not accurate enough (6), or because of the large energy gap introduced in the singleparticle spectrum by partial summations of the perturbative series). The divergence disappears if enough proton-neutron attraction is taken into account in a nonperturbat ive way, for instance by pairing the proton to the neutron. Since the protonneutron interaction is most attractive in the T = 0, S = 1 state, pairing in this state should play an essential role and should be expected to give rise to an energy gap of the order of the deuteron binding energy, which is one order of magnitude larger than the quoted results. Proton-neutron pairing in T = 0, S = 1, s-wave has already been considered (4), yielding the small v~lue of the energy gap of 0.16MeV. The unique way we can see to increase the energy gap is to allow the tensor potential to contribute, by including the d-wave with the s.d coupling. The resulting pair function has the structure of the deuteron wave function and will be referred to as quasi-deuteron. The