A proliferation-inducing ligand (APRIL) is a novel member of the tumor necrosis factor (TNF) family, which is involved in immune regulation. In this study, the cDNA of dog APRIL (dAPRIL) was amplified from dog spleen by RT-PCR. The open reading frame (ORF) of dAPRIL encodes a protein of 250-amino acid, containing a predicted transmembrane domain and a putative furin protease cleavage site like other mammalian APRILs. The amino acid identities between biologically soluble dAPRIL and its pig, human, rabbit and mouse counterparts are 91%, 86%, 88% and 86%, respectively, dramatically higher than most other known cytokines. The result of real-time PCR revealed that dAPRIL is expressed in various tissues and is elevated in thymus and spleen. Recombinant soluble dAPRIL (dsAPRIL) fused with NusA.tag was efficiently produced in Origami B (DE3) pLysS expression host strain. In vitro, purified dsAPRIL was able to co-stimulate the proliferation of dog splenic B cells in response to anti-IgM. These findings indicate that dAPRIL plays an important role in survival/proliferation of dog B cells and provide the basis for investigation on the roles of APRIL in this important domestic species.