Alfin1 cDNA encodes a putative transcription factor associated with salt tolerance in alfalfa (Medicago sativa L). The recombinant protein binds DNA in a sequence specific manner, including promoter fragments of the salt inducible gene MsPRP2. Alfin1 function was tested in transgenic alfalfa under the control of the 35S promoter in the sense and antisense orientations with the endogenous MsPRP2 as a reporter gene. Calli overexpressing Alfin1 were more resistant to growth inhibition by 171 mM NaCl than vector transformed controls, while calli expressing Alfin1 antisense were more sensitive to salt inhibition. Transgenic plants overexpressing Alfin1 in the sense orientation grew well. In contrast, the antisense transgenic plants grew poorly in soil, demonstrating that Alfin1 expression is essential for normal plant development. Transgenic calli and plant roots overexpressing Alfin1 showed enhanced levels of endogenous MsPRP2 mRNA accumalution. However, MsPRP2 mRNA accumulation was also regulated in a tissue specific manner as shown in leaves of transgenics overexpressing Alfin1. These results suggest that Alfin1 acts as a transcriptional regulator in plants and MsPRP2 expression in alfalfa. Alfin1 overexpressing transgenics showed salinity tolerance comparable to one of our salt-tolerant plants, indicating that Alfin1 also functions in gene regulation in salt tolerance.