Abstract: A preparation for activating hemocytes of bivalves for pearl production containing a mitogen as an effective component, and a method for producing pearl through nucleus insertion operation of a bivalve for pearl production which comprises treating a section of the mantle part of a donor to be inserted with a mitogen.By the treatment with the present preparation, hemocytes of the wound site due to the nucleus insertion operation are activated, tissue growth of the section of the mantle part of the donor is accelerated and a good pearl sac is formed around the nucleus. As a result, the number ejected nuclei is decreased and the pearl quality and production amount are enhanced.These effects are further enhanced by using a mitogen treated with an aluminum adjuvant or the like.