Zebrafish (Danio rerio) are an attractive vertebrate model for the molecular dissection of disease mechanisms. Janus kinase (JAK)/signal transducer and activator of transcription (stat) has been defined through studies of cytokine signaling pathways in mammals. Here, we examined the expression level of Jak2a, which is a homolog of mammalian jak2 in zebrafish, by quantitative reverse transcriptase (RT)-PCR, and the peak of mRNA expression occurred at 3.75 hours post fertilization (hpf). The overexpression of Jak2a was proven by real-time Q-PCR and Western blot in 1-4-cell stage embryos injected with 400 ng/mu l full-length jak2a mRNA as well as gfi1.1, gata1, mpo and beta-embryonic hemoglobin as detected by real-time Q-PCR. Moreover, jak2a mRNA significantly increased the GFP(+) population in the transgenic zebrafish lines Tg (gata1: gfp) (uninjected embryos: 17.22 +/- 1.70%; embryos injected with jak2a mRNA: 21.31 +/- 2.11%, p<0.01) and Tg (mpo: gfp) (uninjected embryos: 3.86 +/- 1.94; embryos injected with jak2a mRNA: 6.64 +/- 1.30%, p<0.01) compared with the control group. Thus, our data indicate that Jak2a plays an important role in erythropoiesis and myeloid hematopoiesis.