Immunoglobulin (Ig), lymphoid tissues and Ig-forming cells of the Japanese bullhead shark, Heterodontus japonicus were analyzed biochemically, histologically and immunocytochemically. The serum of Heterodontus contains two Igs with different molecular weights one with 900 K and the other with 180 K daltons. Heavy chains of the two Igs showed an identical molecular weight of 68 K and the same antigenicity, indicating that the two Igs belong to the same class with different molecular structure. Light chains of Heterodontus Igs showed two distinct bands using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, one with the molecular weight of 25 K and the other with 22 K daltons. The latter finding indicates the possible existence of two light chain types in the Heterodontus Igs. White pulp of the spleen appeared as a well-developed lymphoid tissue accompanied large number of Ig-forming cells especially around blood vessels. Massive lymphocytic aggregations were found in the central area of the intestinal valves and certain lymphoid cells were demonstrated to be Ig-forming cells. Ig-forming cells were also observed in the epigonal organ, although the frequency was much less than in the former two tissues. Although the spleen is the major Ig-forming organ in Heterodontus japonicus, the valvular intestine and the epigonal organ also appear to share the function of Ig production.