Interleukin-34 (IL-34) is a multifunctional cytokine that plays an important role in innate immunity and inflammation. Quantitative real-time PCR was used to analyze the expression of IL-34 after bacterial and viral infection. The results showed that IL-34 was constitutively expressed in the tissues of healthy zebrafish and during embryogenesis. The expression of IL-34 was up-regulated in the spleen at 48 and 72 h after intraperitoneal (i.p.) injection with lipopolysaccharide (LPS) or Polyinosinic acid-polycytidylic acid[Poly (I:C)]. The expression of IL-34 was also significantly increased in the intestine (except for the decrease after 24 h LPS stimulation). Infection with Edwardsiella tarda (E. tarda) resulted in increased expression in the intestine at 3, 9 and 24 h post infection, while it was inhibited in the spleen at 6, 9 and 72 h. The IL-34 expression was upregulated in the spleen at 1, 3, 5 and 7d and intestine at 1 and 3 d after i.p. injection with spring viremia of carp virus (SVCV). In ZF4 cells SVCV infection and LPS stimulation enhanced the expression of IL-34. Besides, LPS, IL-1β and IFNφ1 induced the expression of IL-34 in the primary splenic cells. The results suggest that IL-34 is involved in immune defenses against bacterial and viral infection.