Summary: Repeated intradermal inoculation of rabbits with a low-molecular weight synthetic antigen, p-azobenzenearsonate-hexa-l-tyrosine, in Freund's complete adjuvant, induced formation of precipitating anti-p-azobenzencarsonate antibodies. These antibodies were isolated by the use of a specific immunoadsorbent, prepared from a cross-reacting antigen and bromoacetylcellulose. Their physical properties were found to be very similar to those of antibodies, usually formed against high molecular weight antigens. Some indirect evidence was obtained supporting the view that the antigen employed acts on the antibody-forming system independently rather than in combination with proteins of the host.