Serum IgE concentration is measured, according to the Rowe modification of the Mancini technique, in non-atopic patients suffering from various helminthic or protozoal infections. Our results indicate that the IgE level is often raised in parasitosis with prominent tissue phases and remains normal with helminths whose life is restricted to the lumen of the digestive tract. In addition, our observations show that, with helminthic or protozoal infestations, the serum IgE level tends to increase significantly and rapidly following a specific treatment of the parasitosis. Afterwards, the serum IgE level decreases slowly and may return to the normal in a few months. Thus, repetitive evaluations of serum IgE concentrations, before and after therapy, appear to be of a diagnostic value in the cases where a vague clinical picture is suggesting an helminthic parasitosis.