Coelomocyte and brown body production were studied in 4 species of holothurians, Cucumaria miniata, Eupentacta guinquesemita, Parastichopus californicus, and Psolus chitonoides. Available evidence suggests that at least some, if not all, lymphocytes possibly originate from mesenchymal cells in the hemal vessels of holothurians, and later possibly differentiate into hemocytes, amebocytes, and morula cells. Coelomocytes with characteristics which can be arranged in a series intergrading between lymphocytes and hemocytes were observed in the coelomic fluid of Cucumaria and Eupentacta. Coelomocytes with characteristics which can be arranged in a series intergrading between lymphocytes and amebocytes were observed in the coelomic fluid of all 4 species investigated. Lymphocytes in the hemal vessels of these 4 species appear to intergrade with the basophilic morula cells found in the coelomic fluid and in the connective tissues of the mesenteries. Amebocytes containing yellow granular material similar to the yellow granules which compose brown bodies engulf carbon particles injected into the coelomic fluid of the holothurian. Brown bodies containing carbon particles are later produced by these holothurians, indicating that amebocytes may play an active role in the accumulation of brown body material and in the production of brown bodies in holothurians.