The rate of antibody production by the leopard shark, Triakis semifasciata was studied. Two immunoglobulins were elicited, first a 17 S macroglobulin and as intensive immunization continued increasing amounts of 7 S immunoglobulin. Both immunoglobulins were active as phage neutralizing antibodies and contained precipitating antibodies as measured by a microprecipitin test using I125 hemocyanin antigen. Shark immunoglobulins were produced at a slower rate than rabbit or mouse antibodies in response to similar antigenic challenge, and on a weight basis shark antibody precipitated or neutralized less antigen. When studied as antigens using rabbit antishark serum the 7 S and 17 S immunoglobulins appear to be identical. Both are inactivated to the same extent by a reducing agent, dithioerythritol. Dithioerythritol splits the 17 S protein to 7 S units which are antigenically identical with the original 17 S and 7 S proteins. The heavy and light chains derived from each of the immunoglobulins are indistinguishable when tested as antigens with rabbit anti-shark serum.