Use of the common marmoset (Callithrix jacchus) as a non-human primate experimental animal has increased in recent years. Although wasting marmoset syndrome (WMS) is one of the biggest problems in captive marmoset colonies, the molecular mechanisms, biochemical markers for accurate diagnosis and a reliable treatment remain unknown. In this study, as a first step to finding biochemical marker(s) for the accurate diagnosis of WMS, we conducted blood cell counts, including hematocrit, hemoglobin and platelets, and examined serum chemistry values, including albumin, calcium and levels of serum matrix metalloproteinase 9 (MMP9), using a colony of marmosets with and without weight loss. MMP9 is thought to be an enzyme responsible for the degradation of extracellular matrix components and participates in the pathogenesis of inflammatory conditions, such as human and murine inflammatory bowel disease, which, like WMS, are characterized histologically by inflammatory cell infiltrations in the intestines. The values of hematocrit and hemoglobin and levels of serum albumin and calcium in the WMS group were significantly decreased versus the control group. The platelet values and serum MMP9 concentrations were increased significantly in the WMS group compared with the control group. MMP9 could be a new and useful marker for the diagnosis of WMS in addition to hematocrit, hemoglobin, serum albumin and calcium. Our results also indicate that MMP9 could be a useful molecular candidate for treatment.