We define link homology in 4-manifolds, and show that it has a close connection to linking numbers and intersection matrices of 4-manifolds. We also define null-homologous links in 4-manifolds. We give a necessary and sufficient condition for links to be null-homologous in 4-manifolds. This condition implies that for any 4-manifold with second Betti number n, there are (n+2)-component links which are not null-homologous in the 4-manifold.
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