A method is described whereby skin sensitivity to colloidal phenolphthalein and to body-conjugated phenolphthalein was produced. The method consists of feeding phenolphthalein to animals or injecting them with colloidal phenolphthalein and using their serums as antigens to inject into other animals of the same or of a different species. These antigens were found to contain appreciable amounts of a conjugated phenolphthalein and but little or no free phenolphthalein. It is believed that by this method other non-antigenic substances may be made antigenic (9).

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