The invention provides methods for diagnosing B-cell lymphoma in an animal. In particular, the invention provides methods for distinguishing an animal having diffuse large B-cell lymphoma (DLBCL) with an activated B-cell (ABC) phenotype from an animal having DLBCL with a non-activated germinal-center (GC) phenotype. The invention also provides methods for identifying compounds for treating B-cell lymphoma. The invention further provides reagents and methods for determining the amount of miR-155 in sample isolated from an animal. In this regard, the invention provides a set of oligonucleotides for determining the amount of miR-155 in sample isolated from an animal.