The immunotherapeutic properties of a new Pythium insidiosum-vaccine formulation (PIV), was evaluated in 18 horses and 6 dogs with proven pythiosis from different enzootic areas in the United States. All injected horses but one responded with a weak (=29 mm, n=3), a mild (30-90 mm, n=7) or a strong (=100 mm, n=7) inflammatory reactions at the site of injection. Three equines with weak or negative reactions at the injection site were not cured. Seven equines with strong reactions at their injection sites, however, were cured. Six of the eight horses with mild reactions were also cured. The remaining two equines responded at first but both relapsed and finally died of their infections. The PIV cured only two of the six dogs used in this study. The new PIV formulation cured 72% of the equines (P=0.048) and 33% of the dogs with pythiosis. Dogs with chronic disease (greater than two months) did not responded to immunotherapy. The finding of eosinophils, mast cells, IgE and precipitin IgG during pythiosis suggested that a T helper 2 (Th2) subset is in place during this disease. In cured horses, the eosinophilic reaction was substituted by lymphocytes and mononuclear macrophages (Th1). This and previous studies strongly support the hypothesis that an immune-modulation from a Th2 to a Th1 subsets may be in part responsible for the PIV's curative properties.