A traditional Chinese Medicine (TCM) formula, Guo Min Kang (GMK), has been used in clinics in China for allergic diseases, including type I immediate hypersensitivity, a potentially fatal disease, but its modulatory mechanism remains elusive. The aim of this study was to investigate the modulatory mechanisms of GMK in a mouse model of Ag-induced anaphylaxis. Ag (conalbumin) sensitized mice were treated with either PBS (sham) or GMK before (schedule A) or during (schedule B) sensitization, and various anaphylactic parameters were measured following Ag challenge, including symptom score, cutaneous hypersensitivity response, mast cell degranulation, plasma histamine levels and the levels of specific IgE and T-cell responses. Systemic anaphylaxis was investigated in mice immediately following Ag challenge, and the results showed that GMK-treated mice from both treatment schedules A and B showed significantly reduced symptom scores when compared with the sham-treated group. The reduction in symptom score was associated with a significant reduction in the level of Ag-induced cutaneous immediate hypersensitivity. Also, GMK was able to suppress Ag-induced IgE production and T-cell responses, while it spares mitogen (Con A)-induced T-cell response. Further, treatment of mice with GMK abrogated the levels of Ag-induced histamine release and significantly reduced the number of degranulated mast cells. No effect of GMK was observed on the levels of total IgE and plasma histamine in naive mice. These results provide a basis for the modulation effect of GMK and suggest a potential utility of GMK as a prophylactic and therapeutic agent.