Conditions for transmission of a pi-conjugated molecular conductor are derived within the source and sink potential approach in terms of numbers of nonbonding levels of four graphs: The molecular graph G and the three vertex-deleted subgraphs obtained by removing one or both contact vertices. For all bipartite and most nonbipartite G, counting nonbonding levels gives a simple necessary and sufficient condition for conduction at the Fermi level. The exceptional case is where G is nonbipartite and all four graphs have the same number of nonbonding levels; then, an auxiliary requirement involving tail coefficients of the four characteristic polynomials must also be checked.