Activity of antigen were studied after different concentrations of formalin was added to antigen for various period of time. Experiments were performed with precipitin reactions, complement fixation tests, and anaphylaxis. Following results were obtained. There were found exaggerated reactions with increase of period during which time antigen was exposed to formalin. Activity of antigen was reduced when concentrated formalin was used as experimental reagent, while increase of activity was noticed in the case of diluted formalin. A peak of activity of antigen was found when 0.001 per-cent formalin had been reacted to antigen at 37°C for 24 hours.