A new cellular growth factor termed augmenter of liver regeneration (ALR) has been crystallized. ALR has been shown to have a proliferative effect on liver cells while at the same time producing an immunosuppressive effect on liver-resident natural killer cells and liver-resident mononuclear leukocytes. In addition, ALR appears to play an important role in the synthesis and stabilization of mitochondrial gene transcripts in actively regenerating cells. ALR crystals diffract to beyond 2 Å resolution and belong to space group P21212, with a = 125.1, b = 108.1 and c = 38.5 Å. Based on four molecules per asymmetric unit, the Matthews coefficient is calculated to be 2.16 Å3 Da−1 which corresponds to a solvent content of 43%.