RESEARCH into intermetallic superconductors has recently been reinvigorated by the discovery of superconductivity at 23 K in a Y-Pd-B-C quaternary alloy(1) (tying with the long-standing intermetallic record held by Nb-3 Ge (ref. 2)), and at lower temperatures in other quaternary boro-carbide alloys based on Ni, Pd and Pt3-6. The crystal structure of these new intermetallic superconductors, typified by LuNi2B2C (ref. 7), consists of one square layer of rock-salt-type LnC (where Ln stands for a rare-earth element) alternating with one layer of transition-metal boride tetrahedra. Here we report superconductivity at 12-13 K in a new quaternary intermetallic system, lanthanum nickel boro-nitride. We have determined the formula and crystal structure of superconducting La3Ni2B2N3, and also of a related non-superconducting phase LaNiBN. The crystal structure of La3Ni2B2N3 is related to that of LuNi2B2C, but consists of three rock-salt-type LaN layers alternating with tetrahedral Ni2B2 layers, an arrangement considerably more two-dimensional than is found for the superconducting boro-carbides. These discoveries strongly suggest that a large number of unusual superconducting intermetallic phases are get to be found, based on materials more complex than have previously been considered as candidates for superconductivity.