1. The alcohol-soluble extract of heterogenetic antigen, which possesses the specific chemical structure of the entire antigen, has a detectable but generally very slight power to increase the amount of heterogenetic antibodies, when injected into rabbits. 2. This substance can be transformed into an efficient antigen by mixing it with protein solutions such as diluted normal serum. 3. Such mixtures are considerably more active than the same substances injected separately. Therefore, the effect of the serum is probably due to the formation in vitro of a loose compound between the alcohol-soluble substance and protein, the compound acting as a complete antigen. 4. It may be supposed that there exists a group of natural antigens which are built up of one specifically reacting part that is almost or entirely devoid of antigenic properties, and another part, a protein, responsible for the immunizing effect.