TCZ will be administered as a single dose (8mg/kg) intravenously. Eligible subjects will be inpatients at the Children's Hospital of Philadelphia (CHOP) main campus. 10 subjects with HLH will be enrolled. All subjects will be initiated on standard HLH-directed treatment. Cytokine levels [including serum interferon (IFN-γ) and interleukin (IL-6)] will be monitored, in addition to other laboratory and clinical markers of HLH disease activity.